#!/usr/bin/perl
use warnings;
use strict;

my $string = 'abc';
my $pattern = 'B';
my $replacement = 'X';
my $flags = 'i';

$string =~ s/(?$flags:$pattern)/$replacement/;
print $string;  # aXc

##</code><code>##

eval "s/\$pattern/\$replacement/$flags"