Imager can do this quite easily, it has the "data" option instead of "file".

#!/usr/bin/perl
use warnings;
use strict;
use Imager;

my $file = shift || die "Need File $!\n";

my $pngdata;
open (FH,"< $file");
read( FH, $pngdata, -s FH );
close FH;

# Convert image  
my $img = Imager->new;
$img->read(data=>$pngdata, type=>'png')
           or die "Cannot read: ", $img->errstr;

#See perldoc Imager::Transformations 
my $rot20 = $img->rotate(degrees=>20);

$rot20->write(file=>"$file-rot20.png", type=>'png')
           or die "Cannot write: ",$rot20->errstr;

# of course you can write to "data" instead of file here. 
# just use "data=>$output_png"
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