#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    chomp;
    while( m/([[:upper:]](\.\s|\.|\s|)(?:[[:upper:]](?:\2|$))+)/g ) {
	my $acronym = $1;
	print qq("$acronym"\n);
    }
}

exit(0);

__DATA__
L F and LF and L.F. and L. F. and not L, F.
some HTML some XML.
or X.H.T.M.L. or X. H. T. M. L. or even X H T M L
but not U and I,
or You and I.
...