i was poking around with some code in the recent image size pure perl thread and came up with some similar code.

use strict;

my @test = (
        [ '^GIF8..(....)' => sub {
                return ( GIF => unpack("SS", $_[1]) );
                }],
        [ '^^\xFF\xD8.{4}JFIF' => sub { 
                return ( JPEG => unpack("nn", $1)) if ($_[0] =~ /\xFF\
+xC0...(...
.)/s);
                }],
        [ '^\x89PNG\x0d\x0a\x1a\x0a' => sub {
                return ( PNG => unpack("NN", $1)) if ($_[0] =~ /IHDR(.
+{8})/s);
                }],
        [ 'BM.{16}(.{8})' => sub {
                return ( BMP => unpack("LL", $_[1]) );
                }],
);

sub x_image_size {
        return unless $_[0];
        foreach my $test (@test) {
                my @arg;
                return ($test->[1]($_[0], @arg)) if (@arg = $_[0] =~ /
+$test->[0]/s);
        }
}
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