if ($string =~ /^(Trig|Cos|Sin)$/)
{
 # dann mach was
}

use Quantum::Superpositions;
if ($string eq any(qw(Trig Cos Sin))) { ... }

if ( grep { $string eq $_ } qw( Trig Sin Cos ) ) {
# ...
}

use List::Util qw( any );
if ( any { $string eq $_ } qw( Trig Sin Cos ) ) {
# ... 
}

if ($string ~~ ['Trig', 'Cos', 'Sin'])

":Sin:Cos:Trig:" =~ /:$string:/

if mystring in ('Trig', 'Cos', 'Sin'):
     # dann mach was

#!/usr/bin/perl

use strict;
use warnings;

sub isin {
    my $a = pop;
    for (@_) {
        if ($_ eq $a) {
            return 1;
        }
    }
    return 0;
}

my $string = "Trig";
if(isin(('Trig', 'Cos', 'Sin'), $string)) {
    print "Is in.\n";
}