n a group of html files in a single directory I went to change all instances of 2003 to 2004 in each file.

There are many ways of skinning that cat and the only one who won't be happy will be the cat.

One way:

#!/usr/bin/perl -w 
#############################################
# useage: update.pl PATH
#   where PATH is the full path of the directory 
#   we want to fix the files in. 
use strict;
use Tie::File;

exit(0) unless $ARGV[0];

chdir $ARGV[0]
   or die "Cound not chdir to " . $ARGV[0];
#
# Create your own messages for above.

my @files=glob("*.html");

foreach my $file(@files){
   my @ry=();
   tie @ry,"Tie::File",$file 
     or die "Could not open $file";
   $_=~s/2003/2004/g foreach @ry;
   utie @ry;
}
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Just one of many, many, ways of doing this.