Re: Fastest Search method for strings in large file modified to print whole "\n" dilimited records to stdout:

#! perl -slw
use strict;
use List::Util qw[ max ];

our $BUFSIZE ||= 2**16;

my @needles = qw[
    12345
    67890
];

my $regex = '(?:' . join( '|', map quotemeta, @needles ) . ')';
my $maxLen = max map length, @needles;

open FILE, '<', $ARGV[ 0 ] or die "$ARGV[ 0 ]: $!";

my( $toRead, $soFar, $offset ) = ( $BUFSIZE, 0, 0 );
while( my $read = sysread FILE, $_, $toRead, $offset ) {
    if( m[$regex] ) {
        while( m[^([^\n]*$regex[^\n]*$)]mg ) {
            print $1;
        }
    }
    $soFar += $read;

    my $len = length() - rindex $_, "\n";
    substr $_, 0, $len, substr $_, -$len ;
    $offset = $len;
    $toRead = $BUFSIZE - $len;
}
[download]

On my system, performance tails off sharply with BUFSIZEs above 2**16.