Use of uninitialized value $_ in pattern match (m//)

stroke has asked for the wisdom of the Perl Monks concerning the following question:

Hi - can't work this one out, which is frustrating me !

Trying to grab a subset of data from the data input - essentially, each network element and once I have that network stored in the array, check (grep) if the array contains the string 'dhcp-range', and if it does - do stuff.

I think I'm getting the 'Use of uninitialized value $_' error on the 2nd & 3rd end of network data lines ('</ip4-network>'), as those network elements stored in the array don't contain the 'dhcp-range' string. But ... I don't get why it's throwing the error !

On a side note, the data is XML, and I tried using XML::Simple to do stuff, but i want to diff two files in the end, and XML::Simple changed the data structure, so i reverted to this way instead

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;
use 5.010;

# declare
my @net_configs;

my @data = <DATA>;

foreach my $confLine (@data) {

    # find start of a network, push to array
    if ( $confLine =~ /<ip4-network range/ )
    {
        say "Found start of network !";
        push( @net_configs, $confLine );
    }

    # push other network lines onto array
    elsif ( @net_configs )
    {
        push( @net_configs, $confLine );

        # check for end of a network
        if ( $confLine =~ /<\/ip4-network>/ )
        {
            say "Found end of network !";

            if ( grep /dhcp-range/, @net_configs )
            {
                say "Found a dhcp-range !";
                foreach my $net_config (@net_configs)
                {
                    print $net_config;
                }
            }
        @net_configs = undef;
        }
    }
}

__DATA__
<data>
    <configuration name="Test">
        <ip4-network range="10.2.2.0/24">
            <dhcp-range range="10.2.2.50 - 10.2.2.99"/>
            <dhcp-service-option name="next-server" value="10.1.2.3"/>
            <dhcp-client-option name="tftp-server-name" code="66" valu
+e="10.1.2.3"/>
            <dhcp-client-option name="boot-file-name" code="67" value=
+"boot.com"/>
        </ip4-network>
        <ip4-network range="10.2.4.0/24">
            <ip4-address address="10.2.4.2" name="10.2.4.2" state="dhc
+p-reserved" mac="AA-BB-CC-DD-EE-FF"/>
            <dhcp-service-option name="next-server" value="10.1.2.3"/>
            <dhcp-client-option name="tftp-server-name" code="66" valu
+e="10.1.2.3"/>
            <dhcp-client-option name="boot-file-name" code="67" value=
+"boot.com"/>
        </ip4-network>
        <ip4-network range="192.168.1.0/24">
            <dhcp-service-option name="next-server" value="10.1.2.3"/>
            <dhcp-client-option name="tftp-server-name" code="66" valu
+e="10.1.2.3"/>
            <dhcp-client-option name="boot-file-name" code="67" value=
+"boot.com"/>
        </ip4-network>
    </configuration>
</data>
[download]

Output:

Found start of network !
Found end of network !
Found a dhcp-range !
        <ip4-network range="10.2.2.0/24">
            <dhcp-range range="10.2.2.50 - 10.2.2.99"/>
            <dhcp-service-option name="next-server" value="10.1.2.3"/>
            <dhcp-client-option name="tftp-server-name" code="66" valu
+e="10.1.2.3"/>
            <dhcp-client-option name="boot-file-name" code="67" value=
+"boot.com"/>
        </ip4-network>
Found start of network !
Found end of network !
Use of uninitialized value $_ in pattern match (m//) at zTweak.pl line
+ 32, <DATA> line 21.
Found start of network !
Found end of network !
Use of uninitialized value $_ in pattern match (m//) at zTweak.pl line
+ 32, <DATA> line 21.
[download]

Thanks!

Comment on Use of uninitialized value $_ in pattern match (m//) Select or Download Code

Replies are listed 'Best First'.
Re: Use of uninitialized value $_ in pattern match (m//) by toolic (Bishop) on Mar 09, 2016 at 17:45 UTC
You should delete all elements of the array each time through the loop, not assign the value `undef` to the 1st element. Change: `@net_configs = undef;` [download] to: `@net_configs = ();` [download] Tip #4 from the Basic debugging checklist: Data::Dumper	[reply] [d/l] [select]
Re^2: Use of uninitialized value $_ in pattern match (m//) by perlfan (Parson) on Mar 09, 2016 at 17:52 UTC
For that matter, initialize `my @net_configs = ();`	[reply] [d/l]
Re^3: Use of uninitialized value $_ in pattern match (m//) by AnomalousMonk (Archbishop) on Mar 09, 2016 at 18:24 UTC
... initialize `my @net_configs = ();` Why? What would `@net_configs` look like if one were to depend on the default initialization of a newly-created lexical array? Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: Use of uninitialized value $_ in pattern match (m//) by poj (Abbot) on Mar 09, 2016 at 18:16 UTC
XML::Simple changed the data structure With an xml parser like XML::Twig you can build any data structure you want `#!/usr/bin/env perl use strict; use warnings; use XML::Twig; my $t = XML::Twig->new( twig_handlers => { 'ip4-network' => \&network, }, pretty_print => 'indented', ); my $xml = do{local $/;<DATA>}; $t->parse($xml); sub network { my ($t,$e) = @_; my $range = $e->first_child('dhcp-range'); if ( defined $range ){ # do stuff print $range->att('range')."\n"; }; }` [download] poj	[reply] [d/l]
Re: Use of uninitialized value $_ in pattern match (m//) by stevieb (Canon) on Mar 09, 2016 at 17:53 UTC
`@net_configs = undef;` assigns an `undef` as the first and only element in the `@net_configs` array, so next loop through, it triggers the `elsif`, because the array is not empty. Instead, `undef` the array as such: `undef @net_configs;` [download] ...or as others have stated, just re-initialize the array.	[reply] [d/l] [select]
Re: Use of uninitialized value $_ in pattern match (m//) by stroke (Acolyte) on Mar 09, 2016 at 19:09 UTC
Perfect. Thanks. I was using Data::Dumper, but just couldn't see the simple mistake I was making. I'll take another look at the XML processing with XML::Twig - thanks	[reply]