Hey, I'm a physicist, too. Its OK :)

Here I create a flag to detect the first line:

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

my $rest = $spice_str;
my @text;
my $first = 1;
while ( $rest ne '' ) {
    $rest =~ /(.{1,25}[^a-zA-Z0-9.])/;
    push @text, ($first ? $1 : "+$1");
    $rest = $';
    $first = 0;
}

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