#!/usr/local/bin/perl -w

use strict;

my $file = "c:/test.txt";

open(F,"+<$file") or die "Could not open $file: $!";
my @file = <F>;
s/date/localtime/eg for @file;
seek F,0,0   or die "Could not seek in $file: $!";
truncate F,0 or die "Could not truncate $file: $!";
print F @file;
close F;