The segfault is caused by the fill function. Your image is 25 x 25 pixels; the coordinate system starts at 0 and ends at 24, so 25 is just one position too much. A fence-post error, if you will.

Here's the new version of your code, cleaned up slightly (I had nothing better to do. Sue me):

#!/usr/local/bin/perl -w

use strict;
use GD;

my %arcdata = (
        topRight =>     [ 0,  24, 50, 50, 270, 360 ],
        topLeft =>      [ 24, 24, 50, 50, 180, 270 ],
        bottomRight =>  [ 0,  0,  50, 50, 0,   90  ],
        bottomLeft =>   [ 24, 0,  50, 50, 90,  180 ]
);

for my $orientation ( keys %arcdata ) {
        my @insertdata = @{ $arcdata{$orientation} };

        my $img = new GD::Image (25, 25);

        my $white = $img->colorAllocate(255,255,255);
        $img->transparent($white);

        my $black = $img->colorAllocate(0,0,0);
        $img->arc(@insertdata, $black);

        my $red = $img->colorAllocate(255,0,0);

        $img->fill( @insertdata[0,1], $red);

        open (IMAGEOUT,">$orientation.png") || die "Can't write to $or
+ientation.png: $!\n";
                binmode (IMAGEOUT);
                print IMAGEOUT $img->png();
        close (IMAGEOUT);

        print "wrote $orientation.png\n";
}
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