I rarely define such subroutines without also defining the inverse. And once you have the inverse it seems like the most natural thing to do is use it to set up a straight forward table-driven version of the required routine:

#! perl -sw
use 5.010;
use strict;

use constant _1to9       => [ '', qw[ I II III IV V VI VII VIII IX ] ]
+;
use constant _10to90     => [ '', qw[ X XX XXX XL L LX LXX LXXX XC ] ]
+;
use constant _100to900   => [ '', qw[ C CC CCC CD D DC DCC DCCC CM ] ]
+;

sub dec2roman {
    my $dec = shift;
    die "Bad input' unless $dec =~ m[^[0-9]+$] and $dec < 4e3;
    my @decDigits = split '', $dec;
    my $roman = '';
    $roman .= 'M' x        shift @decDigits   if @decDigits == 4;
    $roman .= _100to900->[ shift @decDigits ] if @decDigits == 3;
    $roman .= _10to90->  [ shift @decDigits ] if @decDigits == 2;
    $roman .= _1to9->    [ shift @decDigits ] if @decDigits == 1;
    return $roman;
}

sub roman2dec {
    state %roman2dec;
    %roman2dec = map{ dec2roman( $_ ), $_ } 1 .. 3999 unless %roman2de
+c;

    my $roman = uc shift;
    return $roman2dec{ $roman } if exists $roman2dec{ $roman };
    die "Input '$roman' out of range (1..3999 (roman) inclusive)";
}


for my $r ( qw[ XLII LXIX mi MMMCMLXXXVIII ] ) {
    printf "%15s : %d\n", $r, roman2dec($r);
}
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