The value of s/// is a boolean indicating whether a replacement was made.

Err, no, not really. It's actually the number of replacements made. Thus:

# given
$x = 'abacad';

# then
$x =~ s/a/z/;  # returns 1
$x =~ s/a/z/g; # returns 3
$x =~ s/x/z/;  # returns empty string, not undef
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... although it can be treated quite nicely in a boolean manner :)