use strict;
use warnings;
use GD;

my $outImage = GD::Image->new(1000,1000);

# Assume this image is 500x500 pixels
my $imageTopLeft = GD::Image->newFromPng('/path/to/file/top/left.png');

$outImage->copyResized($imageTopLeft,     # Src Image;
            0,0,                    # $dstX,$dstY,
            0,0,                    # $srcX,$srcY,
            500, 500,               # $destW,$destH,
            500, 500 );             # $srcW,$srcH,

# Assume this image is 200x300 pixels
my $imageBottomRight = GD::Image->newFromPng('/path/to/file/bottom/right.png');

$outImage->copyResized($imageBottomRight, # Src Image;
            800,700,                      # $dstX,$dstY,
            0,0,                          # $srcX,$srcY,
            200, 300,                     # $destW,$destH,
            200, 300 );                   # $srcW,$srcH,

open OUTIMAGE, ">", '/path/to/output/filename.png' or die "Error, unable to write output $!";
binmode OUTIMAGE;
print OUTIMAGE $outImage->png;
close OUTIMAGE;