Thank you all for your answers.

Now I wrote the following code. It is a bit longer, but more readable I think. And so I'm sure that it really is doing what I want. And I also wrote a function which is doing the way back.

use strict;
use warnings;

my @time_str_list = ( "abc", "1abc", ":", ":,", "5:29,11", "29,11", "2
+9:00", "29:00,3", "29:00,303", "9:13,07",  ",11", "0:11", "5:29" );
my $seconds;

for my $time_str ( @time_str_list )
{        
    &convertTimeStrToSeconds($time_str, \$seconds);
    next unless defined $seconds;
    print("$time_str\n");
    print("$seconds\n");
    
    &convertSecondsToTimeStr($seconds, \$time_str);
    next unless defined $time_str;
    print("$time_str\n");

    print("\n");
}

sub convertTimeStrToSeconds
{
    my ($time_str, $ref_seconds) = @_;
    
    # 1) min:sec,frac  e.g. "5:29,11"
    # 2) sec,frac      e.g. "29,11"
    # 3) sec           e.g. "29"
    # 4) ,frac         e.g. ",11"
    # 5) min:sec       e.g. "5:29"
    # 6) someth. else  e.g. "abc"

    $$ref_seconds = undef; # 6)

    # remove leading and trailing whitespace characters
    $time_str =~ s/^\s+|\s+$//g;

    # valid time string
    if( ($time_str =~ m/^(?<min>\d+):(?<sec>\d+),(?<frac>\d+)$/) ||# 1
+)
        ($time_str =~ m/^(?<sec>\d+),(?<frac>\d+)$/) ||          # 2)
        ($time_str =~ m/^(?<sec>\d+)$/) ||                       # 3)
        ($time_str =~ m/^,(?<frac>\d+)$/) ||                     # 4)
        ($time_str =~ m/^(?<min>\d+):(?<sec>\d+)$/) )            # 5)
        
    {
        no warnings 'uninitialized'; # intention: undef treated as zer
+o
        $$ref_seconds = ($+{'min'} * 60.0) + $+{'sec'} + ($+{'frac'}/(
+10**length($+{'frac'})));    
    }
}


sub convertSecondsToTimeStr
{
    my ($seconds, $ref_time_str) = @_;

    $$ref_time_str = undef;

    if( $seconds > 0 )
    {
        my $min = int($seconds / 60.0);
        my $sec = $seconds % 60.0;
        my $frac = int(sprintf("%.2f", ($seconds - int($seconds))*100.
+0));
        
        $$ref_time_str = "$min:" if( $min > 0 );
        $$ref_time_str .= sprintf("%02d", $sec) if ( ($min > 0) || ($s
+ec > 0) );
        $$ref_time_str .= ",$frac" if ( $frac > 0 );
    }
}
[download]

With the error handling you are right. To set $seconds to -1 is not that good. Error handling is in general for me a difficult thing. I'm not sure what is the best way. Now I'm setting the output parameter to undef. Is this good style? Of course I could print a error message. Or I could return undef. Let's assume this function would be part of an official module. What would be the best error handling?

Thank you and Greetings

Dirk