As posted, takes some text as an argument which is used to bring the wanted window to the front and then an Alt-printscreen is issued. The Windows bitmap is grabbed from the clipboard, interpreted and used to construct a GD Image.

It can then be further manipulated with GD or written to disk in any of the GD support formats: PNG, JPEG, GIF etc.

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use GD;
use Win32::GuiTest qw[FindWindowLike SetForegroundWindow SendKeys];
use Win32::Clipboard;

use constant {
    FSIG => 0, FSIZE => 1, FOFFSET => 4, HSIZE => 5,
    WIDTH => 6, HEIGHT => 7, PLANES => 8, BITSPERPIXEL => 9,
    COMP => 10, ISIZE => 11,
};

sub bmp2gd {
    my $bitmap = shift;
    my @info = unpack 'A2ISSI IIISSII', $bitmap;

    my $gd = GD::Image->new( @info[ WIDTH, HEIGHT ], 1 );
    $gd->alphaBlending( 0 );

    my $offset = $info[FOFFSET];
    for my $y ( 0 .. $info[HEIGHT]-1 ) {
        my @rgbs = unpack 'V*', substr(
            $bitmap, $offset+ ( $info[WIDTH]*$y*4 ), $info[WIDTH] *4
        );

        $gd->setPixel( $_, $info[HEIGHT]-$y, $rgbs[ $_ ] ) for 0 .. $i
+nfo[WIDTH]-1;
    }
    return $gd;
}

my $clip = Win32::Clipboard->new();

my @windows = FindWindowLike( 0, "^$ARGV[ 0 ]" ); #Find it
die "Couldn't find window '$ARGV[ 0 ]" unless @windows;

SetForegroundWindow( $windows[0] );
sleep 1;
SendKeys("%{PRTSCR}");

my $fname = "$ARGV[ 0 ]-@{[ time() ]}.png";
print $fname;
if( my $bitmap = $clip->GetBitmap() ) {
    my $gd = bmp2gd( $bitmap );

    open PNG, '>:raw', $fname or die $!;
    print PNG $gd->png;
    close PNG;
    system 1, $fname;
}
else {
    die "Couldn't get bitmap from clipboard";
}

[download]