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.