Perl: timespan between 2 keywords

timespan.pl

#!/usr/bin/env perl

use File::Basename

$nargs = @ARGV;

sub usage() {
    my $prog = basename($0);
    print "Usage: $prog <start-text> <end-text>\n";
}

if ($nargs != 2) {
    usage();
    exit 1;
}

$start = quotemeta($ARGV[0]);
$end = quotemeta($ARGV[1]);

while(<STDIN>) {
    if (/(\d+):(\d+):(\d+).(\d+).*$start/) { 
        $h1 = $1;
        $m1 = $2;
        $s1 = $3;
        $ms1 = $4;

        $t1 = $1 * 60 * 60 * 1000 + $2 * 60 * 1000 + $3 * 1000 + $4;
    } 
    
    if (/(\d+):(\d+):(\d+).(\d+).*$end/)  { 
        $h2 = $1;
        $m2 = $2;
        $s2 = $3;
        $ms2 = $4;

        $t2 = $1 * 60 * 60 * 1000 + $2 * 60 * 1000 + $3 * 1000 + $4; 
        if ($t1 > 0) { 
            $diff = $t2 - $t1; 
            print "t1=$h1:$m1:$s1.$ms1, t2=$h2:$m2:$s2.$ms2, interval=${diff}ms\n";
        } 
    }
}