if ( $color =~ m/^(red|green|blue)$/ ) { blah };

# a common perl idiom is:
my @colors = qw( red green blue pink fuscia );
my $color_re = join '|', @colors;
$color_re = qr/$color_re/;  # compile RE for efficiency

if ( $color =~ m/^(?:$color_re)$/ ) { blah }