You probably want to use backticks AKA qx:
my $html = `echo $plaintext | txt2html --extract`;
# -- or --
my $html = qx(echo $plaintext | txt2html --extract);
# where the ()s can be any seperator you can use on qw(), etc.
However, you might find it better to do this sort of thing with
IPC::Open2, especially if
$plaintext is going to contain stuff that you don't want to pass to the shell:
use IPC::Open2;
my($read_fh,$write_fh);
my $pid = open2($read_fh,$write_fh,"txt2html","--extract");
print $write_fh $plaintext;
close $write_fh;
my $html = do { local $/; <$read_fh> };
# or you can use a while(<$read_fh>) { ... } loop.
waitpid $pid, 0; # don't leave "zombies"
