Only if you use for (my $i; ... ) the variable has the old value again.

$i = 3;

for (my $i = 0; $i < 30; $i++) {  #  \
    # do something here           #  | scope of inner $i variable
}                                 #  /

print $i, "\n";  # prints 3 because it's the outer variable again
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Regarding your second question, that's really a matter of preference and learning. If you plan to do more with perl, it will pay off to learn common Perl idioms, and use them.

If you have access to a public library, I'd very much recommend the book Programming Perl by Wall, Christiansen and Orwant