$ perl -E 'say "a+b+c" =~ y/+/_/r'
a_b_c

##</code><code>##

$ perl -E 'my ($x, $y, $z) = qw{a+b+c + __}; say $x =~ s/\Q$y/$z/gr'
a__b__c

##</code><code>##

$ perl -E 'my ($x, $y) = qw{++ --}; say for "\Q$x\E$y", "$x\Q$y", "\Q$x$y"'
\+\+--
++\-\-
\+\+\-\-