$SIG{__WARN__} = sub {
    my $w = shift;
    warn $w unless $w =~ /::[ab]" used only once:/;
};

##</code><code>##

my $orig_warn = $SIG{__WARN__};
$SIG{__WARN__} = $orig_warn
    ? sub { my $w = shift; $orig_warn->($w) if $w !~ /::[ab]" used only once/ }
    : sub { my $w = shift; warn $w if $w !~ /::[ab]" used only once/ };

##</code><code>##

my $abwarn = $SIG{__WARN__} // sub { warn shift };
$SIG{__WARN__} = sub {
    my $w = shift;
    $abwarn->($w) if $w !~ /::[ab]" used only once:/;
};