my $re = $left ? qr/ab(..)ef/ : qr/gh(..)jk/;

/$re/ or die("Bad data");
print "Caught $1\n"

##</code><code>##

my $re;
if    (...) { $re = qr/.../ }
elsif (...) { $re = qr/.../ }
elsif (...) { $re = qr/.../ }
else        { die '...' }

/$re/ or die("Bad data");
print "Caught $1\n"

##</code><code>##

my %re_lookup = (
   ... => qr/.../,
   ... => qr/.../,
   ... => qr/.../,
);

/$re_lookup{...}/ or die("Bad data");
print "Caught $1\n"