$pat = '(' . join('|', @patterns) . ')';
$pat = qr/$pat/i;

##</code><code>##

$pat = qr/$pat/gi;
s/$pat/something/;

##</code><code>##

$pat = qr/$pat/i;
s/$pat/something/g;

##</code><code>##

perl -e 'print if /perl/gi'

##</code><code>##

$str = 'nothing but perl can parse Perl';
@match = $str =~ /(perl)/gi;
print for @match;