You can do this with straight length and substr.

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

while (<DATA>) {
  chomp;
  my ($from, $to) = split /\s+/;
    
  my $diff = $to - $from;           # get the difference
  ($from, $to) = ($to, $from) if $diff < 0;   # error checking

  if (!$diff) {
      print "$from\n";
  } else {
      $to = substr($to, length($to) - length($diff))
          if length($diff) < length($from) and
             substr($from, 0, 1) eq substr($to, 0, 1);
      print "$from-$to\n";
  }
}

__DATA__
1   32
4   19
28  39
34  123
321 321
324 329
325 349
340 509
51  1
[download]

And the output -

1-32
4-19
28-39
34-123
321
324-9
325-49
340-509
1-51
[download]

Update: Thanks Abigail-II I have fixed the bug in the code.