You're thinking of my() on statements using modifiers. The problem is avoiding only calling my() sometimes. Then the variable may retain is previous value from earlier loops in a way that is declared off-limits and buggy but never fixed because it would be too much overhead to make it consistent.

Ok

while ( my ( $k, $v ) = each %h ) { ... }
while ( my $line = <> ) { ... }
for ( my $ix = 0; ...; ... ) { ... }
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Not ok

my $... = ... if ...;
... or
    my $... = ...;
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