if( $my_var ne "a" and $my_var ne "b" ) { ...keks... }

# oder
if( $my_var !~ m/^(a|b)$/ ) { ...keks... }

    unless ( $my_var eq "a" or $my_var eq "b" ) { ...keks... }  

    unless ( $my_var eq "a" or $my_var eq "b" ) { ...keks... }  

    if ( $my_var ne "a" and $my_var ne "b" ) { ...keks... }  

    if ( not ( $my_var eq "a" or $my_var eq "b" ) ) { ...keks... }  

$ perl /tmp/compare.pl 
                         Rate regex_alter_catch regex_alter_nocatch regex_class equal
regex_alter_catch   1286220/s                --                -29%        -41%  -63%
regex_alter_nocatch 1803742/s               40%                  --        -17%  -48%
regex_class         2163145/s               68%                 20%          --  -37%
equal               3442791/s              168%                 91%         59%    --

#!/usr/bin/perl
# vi:ts=4 sw=4 et:
use strict;
use warnings;
use Benchmark qw( cmpthese );

my $text = 'a';

cmpthese(
    -1, {
        'equal' => sub {
            return if ( $text eq 'a' );
        },
        'regex_class' => sub {
            return if ( $text =~ m/^[ab]$/ );
        },
        'regex_alter' => sub {
            return if ( $text =~ m/^(a|b)$/ );
        },
        'regex_alter_nocatch' => sub {
            return if ( $text =~ m/^(?:a|b)$/ );
        },
    }
);

                         Rate regex_alter_catch regex_alter_nocatch regex_class equal
regex_alter_catch   1293473/s                --                -17%        -39%  -76%
regex_alter_nocatch 1563926/s               21%                  --        -26%  -71%
regex_class         2104367/s               63%                 35%          --  -61%
equal               5367820/s              315%                243%        155%    --