my $compd_pattern = eval "qr($pattern)" or die $@;

##</code><code>##

my $compd_pattern = qr($pattern);

##</code><code>##

my $pat  = ...;
my $repl = ...;

my $mods = '';
$mods .= 'i' if ...;
$mods .= 's' if ...;
$mods .= 'm' if ...;
$mods .= 'x' if ...;

my $re = qr/(?$mods:$pat)/;

if (...) {
    s/$re/$repl/g;
} else {
    s/$re/$repl/;
}