As you are new to Perl I think this is an excellent opportunity to start learning a couple of useful reflexes:

• look at CPAN first, as already mentioned there is a Roman module, it is even reviewed on this site! So there is no need to re-invent this specific wheel.
• use -w and use strict;, they will save you countless hours of chasing after silly bugs,
• test your code! Get into the habit of always writing tests, write them even _before_ you start coding, then when your code passes all the test you know you are done

So here is a test for your code that shows that... it does not pass! Note that handling IIII as 4 can be considered optional, as it is only used on clock dials:

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

while( <DATA>)
  { chomp;
    s{\s*#.*$}{};        # remove comments
    next unless( m{\S}); # skip empty lines

    my( $roman, $decimal)= split /\t/;
    unless( $decimal == decimal( $roman))
      { warn "error $roman translated to ", decimal( $roman), " instea
+d of $decimal\n"; }
  } 
    

sub decimal
  { $_= shift; # roman
    unless ( m/^
                m{0,3}    # thousands
                (cm|dc{0,3}|cd|c{0,3})    # hundreds
                (xc|lx{0,3}|xl|x{0,3})    # tens
                (ix|vi{0,3}|iv|i{0,3})    # ones
               $/ix) {
    die "That doesn't appear to be a well-formed Roman numeral.\n";
    }

    my $val = 0;
    if (s/cm//i) {$val += 900;}
    if (s/cd//i) {$val += 400;}
    if (s/xc//i) {$val += 90;}
    if (s/xl//i) {$val += 40;}
    if (s/ix//i) {$val += 9;}
    if (s/iv//i) {$val += 4;}
    while (m/m/g) {$val += 1000;}
    while (m/d/g) {$val += 500;}
    while (m/c/g) {$val += 100;}
    while (m/l/g) {$val += 50;}
    while (m/x/g) {$val += 10;}
    while (m/v/g) {$val += 5;}
    while (m/i/g) {$val += 1;}

    return $val;
  }

__DATA__
I    1
II    2
III    3
IV    4
V    5
VI    6
VII    7
VIII    8
IX    9
X    10
XI    11
XIV    14
# add more test here, V, C...
MCM    1900
IIII    4    # for clocks
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