Here is one way:

#!/usr/bin/perl -l
use strict;
use warnings;

my $first_col = '';
while (<DATA>) {
    chomp;
    if (/^library/) {
        $first_col = $_;
    }
    else {
        print "$first_col:$_";
    }
}

__DATA__
library_AND
PIN.A
DELAY_TABLE 1 20 2 40 3 60 4 65 5 75;
DELAY_TABLE 1 10 2 15 3 45 4 70 5 80;
POWER_TABLE 20 30 40 50 80 97 110 20;
POWER_TABLE 3 4 5 6 10 80 90 10 20;
library_BUF
PIN.B
LEAKAGE 1 2 3 4;
TRANSIT 1.1 2 3.4;
[download]

Cheers,
Darren :)