G'day Perl_Programmer1992,

Many of Perl's operations use $_ as a default value. You may have seen print and the for loop used that way:

$ perl -MO=Deparse -e 'for (0..2) { print }'
foreach $_ (0 .. 2) {
    print $_;
}
-e syntax OK
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[See B::Deparse if the code in that example is unfamiliar to you.]

$_ is also the default variable for a "pattern match (m//)". These are equivalent:

/fred/
$_ =~ /fred/
$_ =~ m/fred/
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Relating all of that to your code: by assigning the value of each foreach iteration to $line, you do not assign the value to $_ (either explicitly or implicitly by default); consequently, $_ is undefined when used as the default value in /fred/.

By the way, in case you didn't know, for and foreach are synonymous.