$var1 = ($var =~ s/oo//gir);

##</code><code>##

($var1 = $var) =~ s/oo//gi;

##</code><code>##

my @without_oo = map { s/oo//rig } @with_oo;

# traditional version:
my @without_oo = map { (my $x = $_) =~ s/oo//ig; $x } @with_oo;