Why a regex? For extracting fixed strings, substr() is the handiest way. To find the index from where to extract, we'd use index().

So, combining them gives us:

    my $data   = "... whatever ...";
    my $key    = "START";
    my $length =  140;

    my $text   =  substr $data, index ($data, $key) + length ($key), $
+length;
[download]