I'm going to stick to the glaring stuff. There's more, but beyond this what I can come up with goes toward the "readability vs. performance" issues, rather than empirically good vs. bad code. Also, comments would be nice.

#!/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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hmm... /\D.*/. I assume you want to check for the presence of non-digits in the entry. Remember that .* checks for anything, so you're checking for "a non-digit character followed by 0 or more of anything". Besides the obvious warning about .* (see Death to Dot Star!), you just need /\D*/

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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Then you can go straight to the subroutine convb2d(), and I would pass $binary to it, rather than treating the value as a global:

convb2d($binary)
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In the subroutine, either use shift where you now have $binary, or do my $binary = shift; at the top of the subroutine. here's the complete code (tested):

#!/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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There's more you could do, and I'm sure you could get the whole thing down to 2 lines, but this covers the major areas. Mainly, if you have to jump through that many hoops to do input validation on such a simple problem, remember that since TMTOWTDI, there's a high probability that one of those ways is much simpler.

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.