You can change $/ at any time - even alternating as you read a single file. This example demonstrates the flexibility you have, but note that each I/O operation could be on a different file handle as easily as on the same one.

use strict;
use warnings;

for (0..2) {
    my $line1 = do { local $/ = "\n\n"; <DATA> };
    print "got a line1: \"$line1\"\n" if(defined($line1));
    my  $line2 = do { local $/ = "paragraph"; <DATA> };
    print "got a line2: \"$line2\"\n" if(defined($line2));
}
__DATA__
This is a paragraph
with two lines.

This is another paragraph
with two lines.

This is a third paragraph.
This paragraph
has three lines.
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