# get_last returns an array reference for efficiency,
# so we need to de-reference; hence the @{} around
# the subroutine call 
my @lines = @{ get_last("foo.log", 20000) };

sub get_last {
 my ($filename, $from_last) = @_;
 open FILE, $filename or die "Can't open $filename: $!\n";
 my @lines = <FILE>; #loads the whole thing into memory
 close FILE;
 
 # return the last $from_last lines in an anonymous array
 # $#array_name is the index of the last element of 
 # the array, so @lines[$#lines-20, $#lines] is the
 # last 20 elements of @lines
 [ @lines[$#lines-$from_last, $#lines] ];
}