Maybe the perl i'm using is modified as it seems to work correctly.

No. You can see how its broken by giving a different extension, like this

#!/usr/bin/perl --
use Test::More ;
for my $ext ( qw/ jpg gif bmp PNG / ) {
    my $isItTrue = int( $ext eq "jpg"||"gif"||"bmp");
    is( $isItTrue, 1, "$ext" );
}
Test::More::done_testing();
__END__
ok 1 - jpg
not ok 2 - gif
#   Failed test 'gif'
#   at - line 5.
#          got: '0'
#     expected: '1'
not ok 3 - bmp
#   Failed test 'bmp'
#   at - line 5.
#          got: '0'
#     expected: '1'
not ok 4 - PNG
#   Failed test 'PNG'
#   at - line 5.
#          got: '0'
#     expected: '1'
1..4
# Looks like you failed 3 tests of 4.
[download]

I thought CGI caused undue overhead?

If the overhead bothers you, use CGI::Simple, or better yet, get a better webserver, like plackup

I highly recommend you start with beginning-perl book, or Tutorials