use strict;
use warnings;

#emulate reading from 1st file
my %lookup = ();
while (<DATA>) {
    chomp;
    my $key = (split /_/)[0];
    $lookup{$key} = $_;
}

#emulate 2nd file
my @input = ("I would like to learn this ! .",
            "OK ?");

foreach my $line (@input) {
    my @tokens = map {$lookup{$_} || 'unknown'} split /\s+/, $line;
    print "@tokens\n";
}

1;
__DATA__
I_pron
would_mod
like_v
to_to
learn_v
this_pron
._sent
OK_ok
?_quest