Here is a simple program. Do not just use it, though. If you want to write a Perl program, it's best to learn Perl.

#!/usr/bin/perl

$filename = $ARGV[0];  # filename passed on commandline
$size = (-s $filename) * 8 / 128000 / 60;
($min,$sec) = (int($size/60), $size % 60);
printf "$filename is %d:%02d long\n", $min, $sec;
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How does it work? Look in 'perlfunc' for the -s file test. Look at 'perlop' if you need help with the math operators. Look at 'perldoc -f printf' and 'perldoc -f sprintf' to see how printf works in formatting your time output. japhy