In a subroutine @_ is a list of the arguments to the call. For example...

mysub(1, 2, 3);

sub mysub {
  print "@_\n";
  my $x = shift;
  print "@_\n";
  print "$x\n";
}
#Outputs:
#1 2 3
#2 3
#1
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shift within a subroutine grabs the first argument to the call. So in the example above, 1 will be removed from @_ and assigned to $x.