Repeated applications of index would do it: 

my $foo =
"1234567890123456789012345678901234567890
1234567890123456789012345678901234567890
12345678901234567890123456789012345678901
123456789012345678901234567890123456789012
1234567890123456789012345678901234567890
1234567890123456789012345678901234567890
1234567890123456789012345678901234567890
1234567890123456789012345678901234567890";

my ($this, $prev, @short) = (0,0);
while (@short < 4) {
    $this = index $foo, "\n", $prev;
    push @short, substr $foo, $prev, $this - $prev
        if $this - $prev <= 40;
    $prev = $this + 1;
}
{
    local $, = $/;
    print @short;
}

[download]

I expanded the data to show it's doing the right thing.

After Compline,
Zaxo

In reply to Re: How do I match lines of 40 characters long in a block of text? by Zaxo
in thread How do I match lines of 40 characters long in a block of text? by kleinbiker7

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