use constant CONST_STR => "MYSTRING";
    use constant CONST_ID  => 1;

    my %TAGS = ( CONST_STR, CONST_ID );

    my $fullString = "123456 MYSTRING 123456";
    my $string;

    ($string) = $fullString =~ /[A-Za-z]*/;

    print $TAGS{$string};