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

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

Hi Monks,

I'm a Perl newbie, trying to implement it in one of my projects and I have "Use of uninitialized value $_ in pattern match (m//)" error while executing the following code:

#!/tool/pandora64/bin/perl5.8.8 -w

use strict;
use warnings;


my @verilogfiles = glob '*.verilog';
my ($vf, $of, $i, $j);

print "Define the path for sdf_annotate.v: ";

my $path = <STDIN>;

chomp($path);

#For verilog files
foreach my $b (@verilogfiles)
{
        open $vf, '>>', $b or die "Could not open file '@verilogfiles'
+ $!";

       if($_ =~ /module entity\;/)
        {       
            print $vf "\`include \"$path\"\n";
        }

}
close ($vf);
[download]

Output is:

Define the path for sdf_annotate.v: rwfg

Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1. Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1. Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1. Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1. Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1. Use of uninitialized value $_ in pattern match (m//) at opvrmod.pl line 38, <STDIN> line 1.

I tried the solution suggested in this website. But none worked. Any help?

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

Replies are listed 'Best First'.
Re: Use of uninitialized value $_ in pattern match (m//) by roboticus (Chancellor) on Oct 31, 2017 at 19:24 UTC
srikdatta: You were close, but you're getting the error because you haven't done anything to put a value in $_. It appears like you're wanting to scan through a file, so you'd do something like this: #!/tool/pandora64/bin/perl5.8.8 -w use strict; use warnings; my @verilogfiles = glob '.verilog'; my ($vf, $of, $i, $j); print "Define the path for sdf_annotate.v: "; my $path = <STDIN>; chomp($path); #For verilog files foreach my $b (@verilogfiles) { open $vf, '>>', $b or die "Could not open file '@verilogfiles' +$!"; while (<$vf>) { if($_ =~ /module entity\;/) { print $vf "\`include \"$path\"\n"; } } } close ($vf); [download] The `while (<$vf>) {` loop will load a line from $vf into $_ and execute the loop body, and repeat until there are no more lines in the file. ...roboticus When your only tool is a hammer, all problems look like your thumb.*	[reply] [d/l] [select]
Re: Use of uninitialized value $_ in pattern match (m//) by 1nickt (Canon) on Oct 31, 2017 at 19:37 UTC
Hi, `open $vf, '>>', $b` [download] This opens in the file in append mode, so when you do `while (<$vf>) { if($_ =~ /module entity\;/)` [download] (after adding the `while (<$FH>) { ... }` construct as suggested by syphilis), there are no lines read; the loop is never entered. See open. You should open your file in read/write mode, if the intent of your program is indeed as it appears. `open $vf, '+<', $b or die $!;` [download] (Note: I can't make sense of what you are trying to accomplish. You read in a path from the user and then repeatedly print that path to a verilog file, once for each existing line in the file that matches a certain string. Is that really your intent?) Hope this helps! update: added link to doc and example The way forward always starts with a minimal test.	[reply] [d/l] [select]