Two options are now made available:
-d for heuristic by distance, and -s for heuristic by smell, i.e. the reverse flood fill algorithm.
Enjoy!

#!/usr/bin/perl -w

use strict;
use vars qw($opt_d $opt_s);
use Getopt::Std;

getopts('ds');
my @dir = ([0,-1],[1,0],[0,1],[-1,0]);
my (@map, @mark, @dist);
my (%mouse, %cheese);
my $poss = 1;

# initialize map
while(<>) {
    chomp;
    push @map, [map { tr/#// } split //];
    if (/M/) {
        $mouse{x} = length($`);
        $mouse{y} = $. - 1;
    }
    if (/C/) {
        $cheese{x} = length($`);
        $cheese{y} = $. - 1;
    }
}

# init smell or distances
if ($opt_s) {
    flood($cheese{y}, $cheese{x}, 0);
    die "No route to cheese!" unless defined $dist[$mouse{y}][$mouse{x
+}];
} else {
    foreach my $row (0..$#map) {
        $dist[$row] = [map {($_ - $cheese{x})**2 + ($row - $cheese{y})
+**2} 0..$#{$map[$row]}];
    }
}

sub flood {
    my ($y, $x, $dist) = @_;
    return if defined $dist[$y][$x] and $dist[$y][$x] < $dist;
    $dist[$y][$x] = $dist++;
    for (@dir) {
        flood($y + $_->[1], $x + $_->[0], $dist) unless $map[$y + $_->
+[1]][$x + $_->[0]]; 
    }
}

sub find_cheese {
    my ($x, $y) = @_;
    return if not $poss; 

    $mark[$x][$y] = 1;  
    print "X = $x, Y = $y\n";
    $poss = 0 if ($x == $cheese{x} && $y == $cheese{y});

    for (!$opt_d && !$opt_s ? @dir :
        sort { $dist[$y + $a->[1]][$x + $a->[0]] <=> $dist[$y + $b->[1
+]][$x + $b->[0]] }
        ($opt_s ? grep /./, map { 
        defined $dist[$y + $_->[1]] && defined $dist[$y + $_->[1]][$x 
++ $_->[0]] ? $_ : '' } @dir
        : @dir)) {
        !$map[$y + $_->[1]][$x + $_->[0]] &&
        !$mark[$x + $_->[0]][$y + $_->[1]] &&
        find_cheese($x + $_->[0], $y + $_->[1]);
    }
}

find_cheese(@mouse{'x','y'});
print "Cheese found!!\n" if not $poss;
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