<DATA> exhausts the file handle in the first iteration and returns undef in all the following. Either store its content to an array, or use tell/seek to move back to the beginning of the handle.

BTW, don't use for with <>, use while. Also, use \Q$ip\E to treat the dots literally. 

chomp(my @data = <DATA>);
while (my $confLine = <$in>) {
    if ($confLine =~ /ip4-address address="/) { # No need to backslash
+ ".
        for my $ip (@data) {
            if ($confLine =~ /ip4-address address="\Q$ip/) {
                $confLine =~ s{/>}{update"/>};
            }
        }
    }
    print {$out} $confLine;
}

[download]

Or 

my $data_start = tell DATA;
while (my $confLine = <$in>) {
    if ($confLine =~ /ip4-address address="/) { # No need to backslash
+ ".
        seek DATA, $data_start, 0;
        while (my $ip = <DATA>) {
            chomp $ip;
            if ($confLine =~ /ip4-address address="\Q$ip/) {
                $confLine =~ s{/>}{update"/>};
            }
        }
    }
    print {$out} $confLine;
}

[download]

Update: quotemeta added.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Return to original loop by choroba
in thread Return to original loop by stroke

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