I do not think that there is anything wrong with your string, only with the utility you use to display it. I have recreated your file (named it 1223420.png), displayed it with xxd to show I got it right (ignore windows clrl at end), read that file into a string and converted the first 16 characters of that string to hex with unpack. The content of the string is correct. 
$xxd 1223420.png
00000000: 504b 0304 1400 0900 0800 678d 2546 0000  PK........g.%F..
00000010: 0000 0000 0000 0000 0000 1c00 0000 7363  ..............sc
00000020: 7265 656e 7368 6f74 2d31 3732 2032 3120  reenshot-172 21
00000030: 3234 3220 3634 2e7a 6970 329e 8afc b515  242 64.zip2.....
00000040: 0d0a                                     ..

$type 1223420.pl
use strict;
use warnings;
my $string = <>;
print unpack( 'H32', $string );

$perl 1223420.pl 1223420.png
504b0304140009000800678d25460000

[download]
Also note in you output, that although the offending bytes are printed as four hex characters, the address of the following bytes is correct.
Bill

In reply to Re: RT::Client turns occasional binary characters in to wide characters by BillKSmith
in thread RT::Client turns occasional binary characters in to wide characters by wardmw

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