Was showing my kid some perl and was explaining to him what $_ and regexes were. I ran into something unexpected. I had him create the following to print the first two words of each line from a file:

open (my $fh, '<text.txt');

while (<$fh>) {
  /^([^ ]+) ([^ ]+)/;
  print "$1 $2" . "\n" if $1 && $2;
}
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It seems innocent enough. However, it gave me some unexpected results. If the file contains this:

hello one two three
kjsf
kjsd
kjd
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hello one
hello one
hello one
hello one
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That's because no match is found on lines 2, 3 and 4 so it repeats $1 and $2 from line 1. However, if I change the program to this:

while (my $line = <$fh>) {
  $line =~ /^([^ ]+) ([^ ]+)/;
  print "$1 $2" . "\n" if $1 && $2;
}
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hello one
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So why does matching against $line reset $1 and $2 but matching against $_ does not?