Your string as described ends with a colon. All you need is chop - no need for regex:

use strict;
use warnings;

use Test::More tests => 1;

my $have = 'a:b:';
my $want = 'a:b';

chop $have;

is $have, $want;
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use strict;
use warnings;
use Test::Simple tests=>7;

my $NUMBER = qr/[1-9]\d*/;   # Unsigned integer
sub solution {
    return $_[0] =~ s! ($NUMBER:$NUMBER): !$1//!xr;
}

my @test_cases = (
    #given            expected         description
    ['3:4:',         '3:4//',         'minimal'],
    ['3:4:5:',       '3:4//5:',       'begining of string'],
    ['foo:fum3:4:',  'foo:fum3:4//',  'end of string'],
    ['3456:7890:5:', '3456:7890//5:', 'big integers'],
    ['a:b:',         'a:b:',          'no number'],
    ['3:4foo:',      '3:4foo:',       'misplaced colon'],
    ['3456:0890:a:', '3456:0890:a:',  'invalid integer'],
);


foreach my $case (@test_cases) {
    my ($given, $expected, $description) = @$case;
    my $computed  = solution($given);
    ok( $computed eq $expected, $description);
}
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OUTPUT,

1..7
ok 1 - minimal
ok 2 - begining of string
ok 3 - end of string
ok 4 - big integers
ok 5 - no number
ok 6 - misplaced colon
ok 7 - invalid integer
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I encourage you to modify this solution and/or list of test cases as your requirements evolve.

C:\>perl -wle "$x = 'a:b:'; $x =~ s/:$//; print $x;"
a:b
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require 5.010; #<- from your previous post
use strict;
use warnings;

# meth 1 of ?

my $x = 'a:b';
my @x = split(/:/,$x);
my $y = join(q{//}, @x);
print qq{$y\n};

# meth 2 of ?

my @z = sort{ $a <=> $b} @x;
my $a = join(q{//}, @z);
print qq{$a\n};
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a//b
a//b
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> split is also very handy

And to hammer home this point, many mistake the first parameter of split to be a simple string, when in fact it is a regex! Be sure to read what PATTERN will do for you.

Incidentally, split can also be used as a way to trim.

its at the beginning of a large string that may or may not contain other colons

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! :)

---

Give a man a fish:  <%-{-{-{-<

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