This is what I would do:

my $max = [ $x[ 0 ], $y[ 0 ] ];
for my $i ( 1..$#x ) {
  last if $y[ $i ] <= $max[ 1 ];
  $max = [ $x[ $i ], $y[ $i ] ];
}
print "max at ($max->[ 0 ], $max->[ 1 ])\n";
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This code makes two important assumptions: that your data is ordered in the x-axis and that it is really as smooth as you describe it. If neither of these two assumptions hold, the following would be better:

my $max = [ $x[ 0 ], $y[ 0 ] ];
for my $i ( 1..$#x ) {
  $max = [ $x[ $i ], $y[ $i ] ] if $y[ $i ] > $max->[ 1 ];
}
print "max at ($max->[ 0 ], $max->[ 1 ])\n";
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