I would recommend the idiom:

while (local $_ = <$FH>)
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This is certainly best practice -- it stops the while from mangling the caller's precious $_.

Unfortunately, being a good citizen doesn't prevent some mad person from running amuck with an Uzi and blowing away the firstborn.

Consider:

use strict ;
use warnings ;

my $mock = "I\nII\nIII\nIV" ;
open my $FH, '<', \$mock ;

$_ = 'firstborn' ;

while (local $_ = <$FH>) {
  chomp ;
  print "$_=" ;
  tr/IV/iv/ ;
  amuck() ;
  print "$_ " ;
} ;

if (!defined($_)) { $_ = '*undef*' ; } ;
print "  -- $_\n" ;

sub amuck { $_ = '!!!' } ;
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which gives:

  I=!!! II=!!! III=!!! IV=!!!   -- firstborn

so we managed to keep the firstborn safe, but the loop is littered with bodies.

Of course, without the local $_, ie: the common or garden:

while (<$FH>)
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everybody gets it:

   I=!!! II=!!! III=!!! IV=!!!   -- *undef*

Better is to lexically scope the $_, thus:

while (my $_ = <$FH>)
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so that we can still implicitly use $_ in the loop, AND it now works (hurrah!):

   I=i II=ii III=iii IV=iv   -- !!!

OK. The firstborn has gone, but that serves you right for leaving it unattended outside in the push-chair.