my %rows;
while (<DATA>) {
    my ($key, $c1, $c2, $c3) = split;
    push @{ $rows{$key} }, "${c1}_$c2,$c3";
}

for my $key (keys %rows) {
    print join("\t", $key, @{ $rows{$key} }), "\n";
}

__DATA__
frog-n    as    novelty-n    5.8504
frog-n    be    yellow-n    6.1961
frog-n    be-1    Asia-n    5.0937
frog-n    coord    zebra-n    5.9279
frog-n    coord-1    Canuck-n    6.3363
frog-n    nmod-1    mule-n    4.2881
amphibian-n    success-1    surprising-j    14.6340
amphibian-n    such_as    alligator-n    11.5265
amphibian-n    than    work-n    5.9948
amphibian-n    though    stalk-n    13.2228