I have paste the same question on StackOverflow It is solved.

The following script get warning: "Use of uninitialized value in concatenation (.) or string at C:\tools\test.pl line 17, <DATA> line 1." But the next line of __DATA__ will be processed without any warning and get these: test1b.txt:test test1c.txt:test :test More strange thing is that when I add a line: print "$line:".$'."\n"; The warning disappeared. Anybody have some clues? 

#file: test.pl
#!/usr/bin/perl -w
use strict;
my $pattern='test';
my $output='$&';
while(<DATA>)
{
    chomp;
    my $line=$_;
    chomp($line);
    $line=~/$pattern/;
    #print "$line:".$'."\n";   #why uncommenting this line make the fo
+llowing line pass without no warning.
    my $result="$line:".eval($output)."\n";
    print $result;
}

__DATA__
test1a.txt
test1b.txt
test1c.txt

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In reply to strange thing in perl's eval function(solved)) by a369

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