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 within the loop. 

    print $com "...";          # output

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