I'm surprised that no one has mentioned my favorite reason for using single quotes instead of double quotes (in this discussion or the last): more whitespace. Whitespace is pretty, and ' takes half as many pixels as ". Witness:

my $foo = "fhwgads";
if ( $foo eq "fhwgads" ) {
    print "fh" . "wg" . "ads";
}
[download]

my $foo = 'fhwgads';
if ( $foo eq 'fhwgads' ) {
    print 'fh' . 'wg' . 'ads';
}
[download]

Doesn't the second example look *so* much better? Ahh, the beauty. Maybe I like whitespace a little too much?