Take your pick:

• GD
• ImageMagick
• Imager

Those are the most commonly used ones. There are almost certainly others that I've missed. It's usually worth searching at CPAN before asking questions like that here.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $image = Image::Magick->new;
umask 0022;
my @pics= <*.jpg>;

foreach my $pic (@pics){
   my ($picbasename) = $pic =~ /^(.*).jpg$/;
   my $ok;
   $ok = $image->Read($pic) and warn ($ok);
   my $thumb = $picbasename . '-t.jpg';
   $image->Scale(geometry => '100x100');
   $ok = $image->Write($thumb) and warn ($ok);
   undef @$image; #needed if $image is created outside loop 
   print "$pic -> $thumb\n";
}
[download]

And if you're on Unix/Linux, convert has always worked well for me -- I build web picture galleries from a script that calls convert: it scales pictures to a specific size, as well as creating thumbnails, individual web pages and a table of contents. Simple stuff.

I'm sure you already know, but convert is one of the command line tools that are part of Image Magick. We use convert together with composite to add watermarks. Cool tools!

I did know that -- but if all you want to do is resize an image, getting Perl to build a command line for convert can be enough to get the job done.

Alex / talexb / Toronto

"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds