Doesn't placing a variable declaration in the conditional statement for a conditional loop occasionally lead to strange errors?

It shouldn't (or atleast not as far as I know)

Variable declaration with a statment modifier can have stange effects, usally a lexical not getting cleared as expected, due to my having both runtime and compile time effects, the test only blocks the runtime effect of resetting the variable. This can be used for some interesting obfu but should be avoided in serious code IMO.

# if EXPR is false $foo will still have the last set value for this sc
+ope's (but not other scope's) $foo
my $foo if EXPR; 

#same here if EXPR is false the first time it's tested on this pass.
my $bar while EXPR;
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It's rather common to stick a declaration inside the test of a (BLOCK) loop, often seen with input loops, or looping over both keys and values of a hash at the same time.

while (my $foo = <$fh>) {
    #do something
}
while (my ($key, $value) = each %hash) {
    #do other things
}
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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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While you are correct that accessing key value pairs with each is a little faster this is unlikely to influence runtime in any measurable way as the output bottleneck lies with the OS and disk IO.