>perl -wMstrict -le
"my $para =
   qq{\n\n  \t  \nsentence 1\nsentence 2\n\nsentence    3\n\n  \t\n};
 print qq{[[$para]]};
 ;;
 $para =~ s{ \A \s+ | \s+ \z }{}xmsg;
 print qq{[[$para]]};
"
[[


sentence 1
sentence 2

sentence    3


]]
[[sentence 1
sentence 2

sentence    3]]