$spice_str = "VIN IN VSS PWL ( 0ns 0.0 4ns 2.4 25ns 0.0 50ns 2.4 51ns 0.0 75ns 2.4 76ns 0.0 100ns 2.4 101ns 0.0 )";

$rest = $spice_str;
@text = ();

while ( $rest ne '' ) {
    $rest =~ /(.{1,25}[^a-zA-Z0-9.])/ms;
    push @text, $1;
    $rest = $';
}

foreach ( @text ) {
    print "$_\n";
}

##</code><code>##

$rest = "+ " . $';