ok... Here is my edit...

#!/usr/local/bin/perl -w
use strict;
srand;

guess_routine(); #moved the print and rand into the routine
  # that way you don't have the code duplicated when user 
  # says y for new game...

sub guess_routine {
    my @guesses;
    print "I have a number between 1 and 19.\nTry to guess it.\n";
    my $numb=int(rand(20));
    my $in = input();
    while ($in != $numb){
        print($in > $numb ? 'Too high! ' : 'Too low! '); 
           # moved to trinary operator, just shrinks up
           # code, if else isn't bad
        @guesses = sort{$a <=> $b} (@guesses,$in);
           # you were pushing onto the array, then sorting
           # simply sorting array and value together shoule
           # be more efficient
        $in = input(\@guesses); # created a subroutine for
           # this, since you had similar code in two places.
    }
    print "\nYes, $numb is the number!\n";
    print "So you want another go?\n";
    my $answer=<STDIN>; # no chomp, you check /^y/ chomp unecessary
    if ($answer =~/^y/i){
        guess_routine();
    } else {
        print "\nOK, bugger off then!\n";
        exit 0;
    }
}

sub input {
    my $in;
    print "\nSo far you've guessed @{$_[0]}." if $_[0];
    print "\nWhat is your guess?\n";
    $in=<STDIN>;
    chomp ($in);
    $in
}
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