are you sure about that?

Yes.

>perl -Mre=debug -e"sub f { my $pat = $_[0]; (my $str = 'abba') =~ s/$
+pat//g; } f('b'); f('b'); f('b');" 2>&1 | find /c "Compiling"
1
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Before compiling, it checks if the pattern is the same as the one from the last time the operator was evaluated. If so, it reuses the compiled pattern from the last evaluation.

I am using a rather ancient edition of perl (5.8.6), has this behavivour changed since then?

It's not new, but I don't know how old.