I thought that @_ meant the list passed to the sub, so I used $_ when I passed a single element.

@_ contains the list of arguments for a sub, but you want the first argument of that array, which is $_[0]. $_ is something completely different, as you already know.

There are two common idioms for accessing arguments:

sub foo
{
    my $onlyarg=shift; # @_ is the default argument for shift
    # more code
}
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and

sub bar
{
    my ($huey,$dewey,$louie)=@_;
    # more code
}
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Of course, you can also write

sub baz
{
    my $onlyarg=$_[0];
    # more code
}
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or

sub moo
{
    my $huey=shift;
    my $dewey=shift;
    my $louie=shift;
    # more code
}
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or even

sub ohnononononopleaseno
{
    my $huey=$_[0];
    my $dewey=$_[1];
    my $louie=$_[2];
    # more code
}
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But that is uncommon and has some strange accent. Stick with the first two variants. Having only one argument needs no exception, you can use the list assignment even for a single argument, no need for shift:

sub foo2
{
    my ($onlyarg)=@_;
    # more code
}
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Just remember the parentheses, or else $onlyarg will contain the number of arguments, in this case 1. Not what you want.

Alexander