The simplest (fix) I can think of is to redirect the output of sqlplus to a file, and then read from that file afterwards.

open (SQLPLUS, "|sqlplus -s usernam/pass > result.txt" ) or die "";
print SQLPLUS <<SQL
select * from tab where tnam like 'monks';
SQL
close SQLPLUS;

# read the results
open RESULT, "<result.txt";
$result = do { local $/; <RESULT> };
close RESULT;
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