## capture 10 chars (with advancing), then move back 7:

while ( $dna =~ /(.{10})/g ) {
    print "current window = $1\n";
    pos $dna -= 7;
}

##</code><code>##

## capture next 10 chars without advancing, then advance by 3:

while ( $dna =~ /(?= (.{10}) ) .{3}/gx ) {
    print "current window: $1\n";
}