You may want to look at the 3 parameter 'open'.

open( $com, "+<","/dev/ttyUSB0" ) || die "Cannot open serial port : $!
+\n";
while( 1 )
{   my $in = <$com>;           # input

    # do stuff here ( You should be able to do both input and output w
+ithin the loop. 

    print $com "...";          # output

    if ( $in eq "quit" ) { last; }
}
[download]

The default behavior of a 2 parameter 'open' is output only. Look at the documentation for open for more information. Also look at 'read/write' and 'sysopen/sysread/syswrite' if you need to input/output a specific number of bytes. ( Especially if your going to pad with the "\0" null character. )

Good Luck.