Use zero-width lookahead and lookbehind:

$str = "aaabcabcdebcaabebcb";
while($str =~ /(?<=\w{3})(bc)(?=\w{2})/g){
    print "$-[1],$+[1],$1\n";
}
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update: As my esteemed fellow monk Tankalus reminds me, you'll need to adjust your mechanism for retrieving the exact characters you require.