$_ = 'Ab1Cd2eFg3Hijk4LMN';
/((?=([a-zA-Z]{3,}))[A-Z])/ or die "No match";

print <<"END";
Whole match: \$&: '$&' $-[0] upto $+[0]
Parens around everything matched: \$1: '$1' $-[1] upto $+[1]
Lookahead matched: \$2: '$2' $-[2] upto $+[2]
END