The problem is in the algorithm. Assuming that a<b:

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

my $str = $ARGV[0]||"123-135, 111-119, 127-130, 140-145";
#borrowed from ikegami
print "Original string \n" . $str;
print "\nEnding string \n";
print show($_) for map { [ split(/-/, $_) ] } split(/\s*,\s*/, $str);

sub show {
    my (@anums) = split //,$$_[0];
    my (@bnums) = split //,$$_[1];
    my ($output) = $$_[0] . '-';
    my $switch = 0;
    if ($#anums != $#bnums){
        return $$_[0];
    }
    for(0..$#bnums){
    if(defined($anums[$_])){
          #if same, do nothing
      #if larger, include
        if (($bnums[$_] > $anums[$_])|| ($switch)){
        $output .= $bnums[$_];
                $switch++;
            }
        } 
    }
    return $output . ' ';
}
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