• You are using a global variable $decimal in sub convb2d. It would be cleaner to pass that value as an argument to the routine, such as

  convb2d($decimal);
  sub convb2d {
      my $decimal = unpack 
  ( "N", pack("B32", substr("0" x 32 . $_[0], -32 )));
  print ("Decimal: $decimal\n");
  exit;
  }
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• The naming of your routine, sub convb2d would imply to me that it was going to perform a conversion. However, it actually does the conversion, prints the result, and then exits the program. I would have the routine instead simply perform the conversion, and return the result. Imagine future uses -- would this routine be useful if you then wanted to sum up a group of binary numbers in decimal? It wouldn't... but if you did...

  $decimal = convb2d($decimal);
  sub convb2d {
      return unpack 
  ( "N", pack("B32", substr("0" x 32 . $_[0], -32 )));
  }
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  You no longer need the extra variable and can now use this function in many more ways.
• You don't need '()' around print strings.
• The previous authors suggestion regarding the regular expression is very good as well.

You can simplify the check on the binary number:

^ is the beginning of the string
[01] is a character class matching 0 and 1 only
$is the end of the string

$binary=~/^[01]*$/
should probably use a + instead of a *
$binary=~/^[01]+$/

Well, you seem to have the conversion down good (unless I really don't know what I'm doing), but I think you've got far too much code. :) A lot of your testing (as mirod says) could be simplified. Here's what I came up with:

use strict;

sub bin2dec {
   my $arg = shift;
   return unpack("N", pack("B32", substr("0" x 32 . $arg, -32 )));
}

my ($binary, $decimal);

print "Please enter a binary number: ";
do { chomp ($binary = <STDIN>) } until (length $binary);
die "That's not a binary number, dude...\n" if ($binary =~ /[^01]/);
$decimal = bin2dec $binary;
print "$binary = $decimal\n";
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My test is similar to mirod's, but it works by checking to see if there's anything besides 1 and 0, instead of if everything is 1 or 0. I also have the program wait until something is actually entered on the line... but these things are subject to personal taste, IMO.

You'll notice, though, that my bin2dec sub doesn't use any global variables... you'll want to stay away from doing that except in certain situations, as it makes your code less adaptable later on.

That all being said, though, it's not bad code, it's just difficult to understand with the myriad of unnecessary tests in there. I hope that helps... if you've got any more questions, get an account and /msg me! :)

His Royal Cheeziness

1. not digits  ( $binary =~/\D.*/ )
2. digits if ( $binary =~/\d.*/ ) {
3. binary digits  next if /0/ || /1/;

print("Not a binary number!\n");exit;}
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die ("Not a binary number");
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In a more general view, all of your variable are global. You might want to look at perlsub to see how to pass parameters between subs.

however, there are good things, too!
your code ran with modification!
your code uses strict!
and warnings!
you seem to have a decent grasp of the regex character classes.
the convb2d sub works well.

with a little modification, we get :

#!/usr/bin/perl -w
use strict;

print "\n\nEnter a binary number : ";
my $binary=<STDIN>;
chomp ($binary);
die ("not binary") if $binary =~/[^01]/;
&convb2d ($binary);

sub convb2d {
    my $num2conv = shift;
    my $decimal = unpack
( "N", pack("B32", substr( $num2conv, -32 )));
print ("Decimal: $decimal\n");
exit;
}
#eNd
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these 2 line need to remain the same, you can't remove them. Note that we assign a value to $binary at the same time it gets declared.

print "\n\nEnter a binary number : ";
my $binary=<STDIN>;
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chomp ($binary);
die ("not binary") if $binary =~/[^01]/;
&convb2d ($binary);
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chomp (my $binary=<STDIN>);
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Overall, good job. It takes some time to let perl sink in, but the flexibility gained is worthwhile. Have fun learning and try not to get frustrated.
I tried to stay away from things like formatting or use of whitespace; you've probably got a style down that works well for you, and I'd rather help you learn the language than criticize how many returns you have between lines.
update : typo fix. clarified list

#!/usr/bin/perl -w
use strict;
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so far, so good, -w and use strict will take you far.

my (@eachnum, $binary, $good, $decimal);
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I would lose most of these (see below) and put $binary in the if statement.

print("\n\nEnter a binary number:");
chomp(my $binary=<STDIN>);
if ( $binary =~/\D.*/ ) { 
    print("Not a binary number!\n");exit;}
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Furthermore, my impression is that you want to make sure the entry contains nothing but 1s and 0s, right? With a character class, you can take care of that with [^01], and narrow this whole thing down to a single if. I'd also change the print "errmsg"; exit; to a simple die, or maybe a warn and a retry attempt:

die "Not a binary number" if $binary =~ /[^01]/
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convb2d($binary)
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#!/usr/bin/perl -w
use strict;

#get the input
print("\n\nEnter a binary number:");
chomp(my $binary=<STDIN>);

#make sure the number is only 1s and 0s
die "Not a binary number: $!" if $binary =~ /[^01]/

#get the value and print it to the screen
my $decimal = convb2d($binary);
print "Decimal: $decimal\n";

#do the conversion
sub convb2d
{
 my $binary = shift;
 return unpack 
  ( "N", pack("B32", substr("0" x 32 . $binary, -32 )));
}
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MM

UPDATE: removed $! from die at chromatic's suggestion. It was completely unnecessary, as there is no condition under which it will return usable information.

print("\n\nEnter a binary number:");
chomp($binary=<STDIN>);


if ( $binary =~/\D.*/ ) { 
    print("Not a binary number!\n");
    exit;
    }


if ( $binary =~/\d.*/ ) { 

    @eachnum = split (//, $binary);
    $good = 1;
    
    foreach (@eachnum){
        next if /0/ || /1/;
        print("A binary number contains 1's and 0's!\n");
        $good = 0;    
        last;    
    }

    if ( $good == 1 ) {
           convb2d();
    }
}
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