Hmm, very interesting. Which unix platform is giving you problems? It worked fine on linux and solaris for me. If I had to guess, I'd say that maybe your hardware clock is not in synch with your system time. Another option is to use the  -M file test. It returns the value you're looking for in days. Multiplying that by 24, 60, and then 60 again brings it back to seconds. See below, though, because the two methods don't agree. Well, see if this helps anyway:

#!/usr/bin/perl -w
use strict;
system ("ls");
print "file? ";
chomp (my $file = <>);
my $lockfile_epoch = (stat($file))[9];
my $current_epoch = time();
my $sttime = $current_epoch - $lockfile_epoch; # using stat
print "$sttime\n";
my $fttime = (-M $file)*24*3600; # using file test
print "$fttime\n";
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Pepik

Of all the causes that conspire to blind
Man's erring judgment, and misguide the mind,
What the weak head with strongest bias rules,
Is pride, the never-failing vice of fools.
-- Pope.
(Humble to be American.)