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

# moved subroutine calls to _after_ their declarations, not necessary, but a good
# habit that can avoid nasty bugs in more complicated scenarios.
# this also means that input being an auxilliary sub to guess_routine needs to go
# before it
sub input {
    my $arrayref=shift;
    print "\nSo far you've guessed @$arrayref." 
          if $arrayref;
    print "\nWhat is your guess?\n";
    my $in=<STDIN>;
    chomp ($in);
    $in
}


sub guess_routine {
    my $done=0;
    until ($done) {
    	my @guesses;
    	print "I have a number from 0 to 19 inclusive.\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){
	        print "\nOK, bugger off then!\n";
	        $done=1;
	    }
	}
}


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...