# Standard setup stuff and file handle opening goes here
while (<THE_FILE_HANDLE>) {
    s#pattern#replacement#;
}

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;

# Get and open the input and output files

my $INFname = shift;
open my $INFH, '<', $INFName or die "Can't open '$INFName' for input: $!";

my $OUTFName = shift or die "Missing filename(s)!\n";
open my $OUFH, '>', $OUTFName or die "Can't open '$OUTFName' for output: $!";

# Do the search and replace
while (<$INFH>) {
   s/foo/bar/g;
   print $OUFH $_;
}