For data that is not x-ordered, use the following:

use strict;
my $input_file = 'foobar.txt';
open my $in, $input_file or die "Failed to read $input_file: $!\n";
my @data;
while (<$in>) {
  chomp;
  push @data, [ split ];
}
close $in;

@data = sort { $a->[ 0 ] <=> $b->[ 0 ] } @data;

my @max = $data[ 0 ];
my $last = $max[ 0 ][ 1 ];
for ( 1 .. $#data ) {
  my ( $x, $y ) = @{ $data[ $_ ] };
  if ( $y > $last ) {
    $max[ -1 ] = [ $x, $y ];
  }
  elsif ( $max[ -1 ] ) {
    $max[ @max ] = undef;
  }
  $last = $y;
}

pop @max unless $max[ -1 ];

print "Maxima:\n";
print "($_->[ 0 ], $_->[ 1 ])\n" for @max;
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