It takes the first element of the array that's being passed to it. Example:

#!/usr/bin/perl

go ( "foo", "bar", "baz" );

sub go {
  my ($data) = @_;
  print $data;
}
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This will print "foo".

Update: Maybe this snippet from perldoc perlvar is interesting for you:

@_      Within a subroutine the array @_ contains the
        parameters passed to that subroutine. See perl-
        sub.

my $data = $_[0];
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my ($data1, $data2) = @_;
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Had I posted this, I would have lost all my points! Anyway, here is the explaination:

my = declare a variable.
$data = variable name.
() = similar to shift or pop.
@_ = an array that you populate and send it off to a sub routine or a function.

Example


$var = 'some sort of data';
&My_Sub($var);

sub My_Sub
{
  my($data) = @_;
 Print $data . "\n";
}

output:

some sort of data
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Blackadder

Given that you know what it does, it means:

Instantiate a scalar called $data, put it in a list of one element, then assign to that one-element list the contents of list @_. Because the left-hand side contains only one element, only one element (the first) is copied from @_.

Try reading some documentation.