Correct me if I'm wrong, but localtime does all that conversion/daylight savings/boundary work for you. Wouldn't this work?

use strict;
use warnings;

sub myGetTime {
    my($timezone) = shift;
    $timezone = 0 unless $timezone;
    # convert to seconds
    $timezone = $timezone * 3600 if ($timezone);
    my($time) = time + $timezone;
    return (gmtime($time));
}

print "Local  : " . localtime(time) . "\n";
print "London : " . &myGetTime      . "\n";
print "Houston: " . &myGetTime(-5)  . "\n";
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