Try using the //e modifier with the //g modifier. For instance:

my @array = (1, 2, 3, 4, 5, 6);
my $string = "ababbabaaba";
$string =~ s/a/shift(@array)/ge;

print $string; # prints "1b2bb3b45b6"
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The //e modifier executes whatever is between the last to //'s as code.

elusion : http://matt.diephouse.com