use Text::Balanced qw( extract_bracketed );
my $text = q/%%% rick ran errands . sem(1,    [     word(1001,rick),     word(1002,ran),     word(1003,errands),     word(1004,'.')    ],    [     pos(1001,'NN'),     pos(1002,'VBD'),     pos(1003,'NNS'),     pos(1004,'.')    ] )./;
my ($extracted,$remainder) = extract_bracketed($text,'()');