Not to take anything away from Athanasius post, but being Perl, timtowtdi :) Here's another potential solution.

Instead of looking at it like you have to transform the array into key/value pairs in the hash, you can simply throw the array into the hash directly:

my $tmp = {
    NameTag => $nametag,
    Name    => $name,
    Params  => \@params,
};
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Then:

sub CreateString {
    ( my $self, my $hash ) = @_;

    my $str = "A|"
          . $hash->{'NameTag'} . "|"."Command"."|"         
          . $hash->{'Name'} . "|"
          . join( "|", @{ $hash->{ Params } } );

    return ($str);
}
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Update: In fact, you can also eliminate the need for the $tmp var entirely by calling your sub like this:

my $InputRequest = $Obj->CreateString({
                                NameTag => $nametag,
                                Name    => $name,
                                Params  => \@params,
                            });
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