I use that on NT4 all the time... are you using UNC filenames or a mapped drive? (UNC works). Try a one-liner...

C:\rexec>echo > file.txt
C:\rexec>perl -e "print scalar localtime((stat('file.txt'))[9])"
Wed Aug 18 10:33:50 2004

Most likely, you are running without warnings and are not error-checking your results. My guess is, as you don't show how $file gets filled, that a file with the name in $file does not exist.

You will need to modify your code to track down this error, preferrably as follows:

use strict;
use warnings;
use Carp qw( carp );

sub get_mtime {
  my ($file) = @_;
  if (! -e $file) {
    carp "There exists no such file as '$file'";
    return undef;
  };
  if (! -f $file) {
    carp "There exists '$file' but it isn't a file.";
    return undef;
  };
  my $result = (stat $file)[9];
  carp $!
    if $!;
  $result;
};

my @files = qw(
  c:\\winnt\\cmd.exe
  c:\\winnt\\system32\\cmd.exe 
  c:\\winnt
);

for my $f (@files) {
  printf "'%s' has the mtime %s\n", $f, (get_mtime($f) || 'n/a');
};
[download]

While that code is overly verbose, it should help you much with diagnosing the real cause of your error.