How about a one-liner...

$imagename =~ s/^(.*?)(\.\w*)$/my $temp = $2; ($_ = $1) =~ s!\W!_!g; $
+_ .= $temp/e;
[download]

It deals with your examples plus the most pathological tests I could think of.

#! perl -sw
use strict;

my @filespecs = map{ $_ =~ s/^(.*?)(\.\w*)$/my $temp = $2; ($_ = $1) =
+~ s!\W!_!g; $_ .= $temp/e; $_} <DATA>;

print $_, $/ for @filespecs;

__DATA__
me~swimming.jpg
J.R,ewing.jpg
A¬!£$%^&*()-_+={}[]'@;:~\#\"<>,./|\?.jpg
pic.
[download]

Output

C:\test>199139
me_swimming.jpg

J_R_ewing.jpg

A___________________________________.jpg

pic.


C:\test>
[download]