Hi All, I know this is a 10 year old thread, but I stumbled across it as I was searching for the same thing... Here is my solution:

sub _getJpegDimensions {
    my $path = shift;

         # read first 100kb - this should be enough to get the SOF sec
+tion.
    open my $fh, '<', $path;
    read $fh, my $data, 102400;
    close $fh;
    my @chars = split("", $data);

    my @asciis = map { ord() } @chars;  # convert all chars to ascii v
+alues
    
        # jpg images start with xff xd8, so check this first and remov
+e if found
    if ((shift @asciis == 0xff) and (shift @asciis == 0xd8))
    {
            # loop through each section until the SOF section is found
        while(@asciis) 
        {
            my $val = shift @asciis;
            
                # check if $val is possibly the start of a marker (all
+ markers are 0xffXX)
            if ( $val != 0xff )
            {
                next;
            }
            else 
            { 
                $val = shift @asciis; 

                    # Start of frame marker is usually 0xffc0
                    # check for c0 to c3 to include extended sequentia
+l, progressive and lossless markers
                if (($val >= 0xc0) and ($val <= 0xc3))
                {
                    # SOF block is [0xffc0][ushort length][uchar preci
+sion][ushort x][ushort y]
                    #               [0xffc0][   2 Bytes   ][    1 Byte
+     ][2 Bytes ][2 Bytes ]
                    # note that x is height, y is width
                    my $width = ($asciis[5] * 256) + $asciis[6];    # 
+convert the 2 byte ushort into a decimal number
                    my $height = ($asciis[3] * 256) + $asciis[4];
                    return ($width, $height);
                }
                else
                {
                    # if this isnt an SOF section, skip the rest
                    # section length is immediately after marker
                    # section length includes marker, hence why 2 is s
+ubtracted
                    my $section_length = shift @asciis;
                    if ($section_length - 2 > 0)
                    {
                        splice @asciis, 0, $section_length - 2;
                    }
                }
            }
        }
    }
    else
    {
        return (0,0);
    }
}
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This is called as follows:

my ($width, $height) = &_getJpegDimensions("path to jpg as string");
[download]

Note - this code makes the following assumptions: 1. 100kb is enough to get the SOF header. 2. The .jpg is encoded using Baseline, Extended Sequential, Progressive or Lossless schemes. Hope this helps! Jon