Curly braces are needed around blocks. In the pythonesque code you posted, a left brace is missing whenever the indentation increases, and the right brace is missing whenever it decreases.

Unfortunately, putting the curly braces in doesn't make the code work. The user's input must be repeated in the loop, too, otherwise the code would test the same number again and again.

#! /usr/bin/perl
use warnings;
use strict;

my $im_thinking = int rand 10;
print "Pick a number: ";

my $guess = -1;
while ($guess != $im_thinking) {
    $guess = <STDIN>;
    chomp $guess;
    if ($guess > $im_thinking) {
        print "You guessed too high!\n";
    } elsif ($guess < $im_thinking) {
        print "You guessed too low!\n";
    }
}
print "You got it right!\n";
[download]

Update: You can use last and the ternary operator to make the code even simpler:

#! /usr/bin/perl
use warnings;
use strict;

my $im_thinking = int rand 10;
print "Pick a number: ";

while () {
    my $guess = <STDIN>;
    chomp $guess;
    last if $guess == $im_thinking;
    print "You guessed too ", $guess < $im_thinking ? 'low' : 'high',
        "\nTry again: ";
}
print "You got it right!\n";
[download]