But what if my executable is /bin/foo bar?

Is this what you mean?

my $sh = '/home/kyle/harm/spacey name.sh';
open my $sh_fh, '>', $sh
    or die "Can't write '$sh': $!";
print $sh_fh 'echo "spaces are fun!"';
close $sh_fh;

chmod 0755, $sh
    or die "Can't chmod '$sh': $!";

$sh =~ s/ /\\ /g;
print qq{I shall now: system( "$sh" );\n};
system( "$sh" );
__END__
I shall now: system( "/home/kyle/harm/spacey\ name.sh" );
spaces are fun!
[download]

Or are you talking about something more ambiguous?