How about:

my $rindex = rindex($imagename,'.');
$rindex    = length($imagename) if $rindex == -1;
substr($imagename,0,$rindex) =~ s/\W/_/g;
[download]

It will exclude the extension from the substitution.

Update: Fixed bug for filenames w/o extensions that end in a nonword char.

Update2: Thought I'd golf this one a bit:

substr($file,0,rindex($file,'.')%length"a$file") =~ s/\W/_/g;
[download]

works correctly for $files such as "me~swimming.jpg", "J.R,ewing.jpg" and "abc;;;"

-Blake