#!/your/perl/here
use strict;
use warnings;

my $last_sign;
my $last_y;
my $last_x;
while (<>)
{
    my ($x,$y) = split;
    my $sign = $y <=> $last_y;
    if ( $last_sign <=> $sign )
    {
        print "$last_x $last_y\n";
    }
    $last_x    = $x;
    $last_y    = $y;
    $last_sign = $sign;
}