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#!/usr/bin/perl -w
use strict;

use warnings;

my $data = get_file($jpg);
my @res  = sizeJPG($data);
print "$jpg width $res[0] height $res[1]\n";

my $data = get_file($jpg);
my ($h,$v)  = sizeJPG($data);
print "$jpg width $h height $v\n";

sub sizeJPG {
    return unless $_[0];
    my ( $width, $height );
...
   unpack( "nn", $1 ) if $_[0] =~ /\xFF\xC0...(....)/;
    return ( $width, $height );
}

local $_=shift;

my ($width,$height)=unpack "nn", (/\xFF\xC0...(....)/)[0];

my ($width,$height)=unpack "nn", /\xFF\xC0...(....)/ ? $1 : '';
# or "\0" x 4, maybe?

sub get_file {
    open FILE, $_[0] or die $!;
    binmode FILE;
...
    close FILE;
    return $data;
}

sub get_file {
    local $/;
    open my $fh, '<:raw', shift or die $!;
    <$fh>;
}