It is @_, definitely not $_.

You can refer to a single argument like your $header as $_[0]. If the contents you parse may be delivered in a list,

sub parse_header {
    for (@_) {
        # do this and that to $_, which is set by 'for'
    }
}
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That will work with a single argument, too. Doing it with while would fail if you did not remove elements from @_ as you dealt with them.