G'day samh785,

The special variables $1, $2, etc. are reinitialized on each match. This includes matches done as part of a substitution. After '$mac1 =~ s/:/-/g;', which contains no captures, both $1 and $2 are uninitialized.

Here's a example:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $string = 'abc|def';

if ($string =~ /(\S{3})\|(\S{3})/) {
    print "After first match (in if condition): \$1=$1 \$2=$2";
    my $whatever = 'blah';
    $whatever =~ s/b/c/;
    print "After second match (in substitution): \$1=$1 \$2=$2";
}
[download]

Output:

After first match (in if condition): $1=abc $2=def
Use of uninitialized value $1 in concatenation (.) or string at ./pm_1
+143443_match_vars.pl line 12.
Use of uninitialized value $2 in concatenation (.) or string at ./pm_1
+143443_match_vars.pl line 12.
After second match (in substitution): $1= $2=
[download]

Assigning $2 to $mac2 before attempting the substitution should fix your problem.