So the problem was not as described. The goalpost move count is now at 1. See How to ask better questions using Test::More and sample data.

use strict;
use warnings;

use Test::More tests => 1;

my $have = '111:99:678:foo:{}';
my $want = '111:99678:foo:{}';

$have =~ s/^(\d+:\d+):/$1/;

is $have, $want;
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So how about an example of what you're really talking about? Is it something like '123:45:6789:32:654:87' or what? (Please don't give a 10,000 character long example! :)

> Please don't
what have you done? :)