sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }

##</code><code>##

sub f { my $re = qr/$_[0]/; (my $str = 'abba') =~ s/$re//g; }

##</code><code>##

sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }
f('a')
f('b')

##</code><code>##

sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }
f('b')
f('b')
f('b')

##</code><code>##

sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }
f('a')
f('b')
f('a')
f('b')

##</code><code>##

my $re_a = qr/a/;
my $re_b = qr/b/;
sub f { my $re = $_[0]; (my $str = 'abba') =~ s/$re//g; }
f($re_a);
f($re_b);
f($re_a);
f($re_b);