# open a pipeline in which a perl one-liner feeds
# word-tokenized data from $file to "word_filter";
# the FILT file handle is then used to read the output
# of word_filter (one line per word):

open( FILT, "perl -pe 's/^\\s*//; s/\\s+/\\n/g' $file | word_filter |" )
    or die "subshell: $!";
                          ## note the "\\" in the one-liner
while (<FILT>) {
   # $_ holds one line of output from "word_filter"
   # so do something with that...
}