May I suggest that you convert both times to seconds before you try to compare the times as this makes the comparison much easier?

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;

sub timeLeft
 {my ($Hours, $Minutes, $Seconds) = @_;
  
  my ($hours, $minutes, $seconds) = @{[localtime(time)]}[2, 1, 0];
  my $currentTimeInSeconds = $hours*3600+$minutes*60+$seconds;
  my $targetTimeInSeconds  = $Hours*3600+$Minutes*60+$Seconds;

  $targetTimeInSeconds > $currentTimeInSeconds ? $targetTimeInSeconds 
+- $currentTimeInSeconds : undef
 }

say timeLeft(qw(20 50 00));
[download]

Produces:

158