my $what = "fred|barney";
...
if ($_ =~ ($what){3}) { ... }

##</code><code>##

if ($_ =~ $what{3}) { ... }

##</code><code>##

if ($_ =~ //) { ... }

##</code><code>##

if (1) { ... }

##</code><code>##

my $what = "(fred|barney){3}";
...
if ($_ =~ $what) { ... }

##</code><code>##

if ($_ =~ /($what){3}/) { ... }
....
if ($_ =~ /$what/) { ... }

##</code><code>##

if (/($what){3}/) { ... }
...
if (/$what/) { ... }