First, s/// doesn't compile the pattern more than once per pass of /g.

Compiles at most once per call to f, even if it replaces twice:

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

sub f { my $re = qr/$_[0]/; (my $str = 'abba') =~ s/$re//g; }
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Now to explain the "at most" bit.

Compiles twice:

sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }
f('a')
f('b')
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Compiles once (since Perl notices the pattern doesn't change):

sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$pat//g; }
f('b')
f('b')
f('b')
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Finally, how can you avoid excessive recompiling?

Compiles four times (since the pattern keeps changing):

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

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);
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