#! perl -slw
use strict;

my $re_etime = qr[
    # Init a var to hold our value.
    (?{ our $ETIME = 0 })

    # The whole line is optional
    # Capture 2 digits followed by - if present
    # What we captured $ETIME = ($^N) or 0 * 24

    (?: (\d{2}) (?{ $ETIME = 24 *       $^N||0  }) - )?

    # Optional \d{2}: if present, add them in, convert to seconds

    (?: (\d{2}) (?{ $ETIME = 60 * ($ETIME + $^N||0) }) : )?

    # Compulsory 2 digits , add em in and convert to seconds
        (\d{2}) (?{ $ETIME = 60 * ($ETIME + $^N)    })

        : # Compulsory colon

    # And the seconds.
        (\d{2}) (?{ $ETIME += $^N               })
]x;

for my $test ( '25:41', '19:25:41', '11-19:25:41' ) {
    our $ETIME; # Gain access to our var

    #If the match succeeds, $ETIME has our value.

    print "$test => $ETIME" if $test =~ $re_etime;
}
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Personally, I find the uncommented version clearer and more readable. The comments just seems to muddy things up.