Notice also that you're processing two strings even if they're not formatted correctly (that is, match neither of the =~ statements, the second of which should be an else.

Thanks apl ! Here is the updated code (still does not work), I get nothing from the print.

#! perl -slw
use strict;
use diagnostics;
use Data::Dump qw[ pp ];

sub by_hour{
    my ($a,$b) = @_;
    my $h_a;
    my $mn_a;
    my $h_b;
    my $mn_b;
    
    
    print STDOUT "a is $a\n";
    print STDOUT "b is $b\n";

    
    my $res;
    
    if ($a =~ /(\d\d):(\d\d)/){
            $h_a = $1;
            $mn_a = $2;
        }
    else{
        $res = 0;
    }
    if ($b =~ /(\d\d):(\d\d)/){
            $h_a = $1;
            $mn_a = $2;
        }
    else{
        $res = 0;
    }    

    print STDOUT "h_a is $h_a\n";
    print STDOUT "h_b is $h_b\n";
    print STDOUT "mn_a is $mn_a\n";
    print STDOUT "mn_b is $mn_b\n";    
        
    if ($h_a < $h_b){
        $res =    -1;
        }
    if ($h_a > $h_b){
        $res = 1;
        }
    if ($h_a == $h_b){
        
        if ($mn_a < $mn_b){
        $res = -1;
        }
        if ($mn_a > $mn_b){
        $res =1;
        }
        if ($mn_a == $mn_b){
        $res =    0;
        }
    }
    return $res;    
}

my %in;

while (<DATA>){
m[(\d\d:\d\d) (.+)$] and push @{ $in{ $1 } }, $2;
}

for my $hr ( sort by_hour keys %in) {
    print STDOUT "$hr\n";
}


__DATA__
11:10 A1
12:30 E4
11:30 Z4
15:50 H5
12:02 H6
12:25 B2
11:25 A8
12:30 F3
14:30 E7
12:50 E15
12:55 E16
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