Use opendir and readdir (perldoc -f opendir and the same for readdir)

opendir HTMLDIR, $directory_name or die "Can't open $directory_name: $
+!\n";
while (readdir (HTMLDIR)) {
    push @images, $_) if /\.(png|jpg|gif|tiff)/;
}
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When you're done, @images will contain a list of the files with the extensions jpg, png, gif, or tiff (go wild here =)
UPDATE : d'oh! the loop I wrote above doesn't set $_ as it loops through the directory. Try

foreach ((readdir(HTMLDIR)) {
# stuff
}
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instead.

And, while we're at it, you could do

my @files = grep { /png|gif|jpg|tiff$/ } (readdir(DIR));
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For one-liner goodness (grep { condition } list grabs everything in list that meets condition)
I daresay these may not be as efficient as Adam's solution above.

Philosophy can be made out of anything -- or less