$user_defined_string =~ s/$user_defined_search/$user_defined_replace/eeg;

__END__
before: abcabcabc
after: ---a---bc---a---bc---a---bc

##</code><code>##

my $user_defined_replace = '`whoami`';

before: abcabcabc
after: xxx
bcxxx
bcxxx
bc