Everyone told you how to do it right, but no one told you what you were doing wrong.

my $xcltime = "19:45:34"; # It should print 7:45 PM
my $xcltime2 = "23:35:14"; # It should print 11:35 PM

if (($xcltime =~ /(\d{1,2}):(\d{1,2}):(\d{1,2})/) || ($xcltime2 =~ /(\
+d{1,2}):(\d{1,2}):(\d{1,2})/)) {
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You should probably do a loop over both $xcltime and $xcltime2, to make sure BOTH of them get the treatment.

    my $h1=$1;my $m2=$2; my $s3=$3;
    if ($h1 =~ /13||14||15||16||17||18||19||20||21||22||23||24/) {
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You're confusing Perl's "OR" operator || with the regex's alternation symbol |. When you use a||b in a regex, you're saying "match 'a' or nothing or 'b'".

There's also no point in saying "if $str matches /foo/, substitute 'bar' for /foo/". Just say "substitute 'bar' for /foo/", and if any matches are found, they'll be changed. The idiom s/a/b/ if /a/ is pointless.

         $h1=~s/13/1/;
         $h1=~s/14/2/;
         $h1=~s/15/3/;
         $h1=~s/16/4/;
         $h1=~s/17/5/;
         $h1=~s/18/6/;
         $h1=~s/19/7/;
         $h1=~s/20/8/;
         $h1=~s/21/9/;
         $h1=~s/22/10/;
         $h1=~s/23/11/;
         $h1=~s/24/12/;
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And there's no reason to use regexes here. $h1 is just a number now. This logic should be written as

my $pm = 0;  # assume AM
if ($h1 > 12) {
  $pm = 1;
  $h1 -= 12;
}
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