As discussed here, $_ is automatically localized in a foreach loop. So, for example:

foreach (qw/foo bar/) {
  print;
  print foreach (qw/one two three/);
}
# output: fooonetwothreebaronetwothree
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However, a while loop does not do this automatically:

foreach (qw/foo bar/) {
  print;
  my @list = qw/one two three/;
  print while (shift @list); 
}
# output: foofoofoofoobarbarbarbar
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In order to do this, you need to explicitly localize $_ within the while loop:

 
foreach (qw/foo bar/) {
  print;
  my @list = qw/one two three/;
  print while (local $_ = shift @list); 
}
# output: fooonetwothreebaronetwothree
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Another way to do it:

{ local $_; print while (shift @list); }
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Note that you must have the local declaration enclosed in a block with the while statement, as above. The following example will not work:

# will not localize $_ to the while loop
local $_; print while (shift @list);
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See perlop for more information.