use strict;
use warnings;

my $file = '...';

open my $pngIn, '<:raw', $file or die "Can't open '$file': $!\n";
read $pngIn, my $header, 8;

my ($prefix, $png, $crlf, $ctrlZ, $lf) = unpack "aa3a2aa", $header;

die "Bad header prefix\n" if ord $prefix != 0x89;
die "Bad header type\n" if $png ne 'PNG';
die "Bad header crlf\n" if $crlf ne "\r\n";
die "Bad header Ctrl-Z\n" if ord $ctrlZ != 0x1a;
die "Bad header newline\n" if $lf ne "\n";

while (!eof $pngIn) {
    read $pngIn, my $chunkHeader, 8 or last;
    
    my ($length, $type) = unpack "Na4", $chunkHeader;
    
    read $pngIn, my $body, $length + 4 or die "Truncated chunk\n";
    
    my ($payload, $crc) = unpack "a$length N", $body;
    
    last if $type eq 'IEND';
}