>perl -wMstrict -le
"my $s =
   'there is no FOO stuff i do not want BAR unwanted BAR stuff here';
 ;;
 my $t = $s;
 $t =~ s{ FOO \K .*? (?= BAR) }{}xms;
 print qq{'$t'};
 ;;
 $t = $s;
 $t =~ s{ FOO \K .* (?= BAR) }{}xms;
 print qq{'$t'};
"
'there is no FOOBAR unwanted BAR stuff here'
'there is no FOOBAR stuff here'