All the arguments are in the array @_. In the provided code, the function only has one argument, so it can be referenced as $_[0]. A convenient way of assigning names to your arguments is

sub parse_header {
   my ($header) = @_;
   do this and thia
}
[download]

but some people prefer

sub parse_header {
   my $header = shift;
   do this and thia
}
[download]

Inside a function, shift with no arguments is the same as shift(@_).

If you had more than one argument, the above would look as follows:

sub three_args_func {
   my ($arg0, $arg1, $arg2) = @_;
   do this and thia
}
[download]

sub three_args_func {
   my $arg0 = shift;
   my $arg1 = shift;
   my $arg2 = shift;
   do this and thia
}
[download]

Reference: perlsub