G'day mmittiga17,

Here's another way to do it:

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

local $/ = 'CUSIP';

while (<DATA>) {
    s{\s*$/$}{}m;
    next unless $_;
    s{\n}{ }gm;
    s{\s*$}{};
    say $/, $_;
}

__DATA__
CUSIP    data data data
data data data data
data data data data
CUSIP    data data data
data data data data
data data data data
CUSIP    data data data
data data data data
data data data data
[download]

Output:

$ pm_merge_multiline_with_leader.pl
CUSIP    data data data data data data data data data data data
CUSIP    data data data data data data data data data data data
CUSIP    data data data data data data data data data data data
[download]