my $newrate = 0;
### AVOID ANY NASTY DIVISION-BY-ZERO ERRORS WITH AN EVAL...
eval
{
my $newrate = ($first_currency / $second_currency);
};

### IF EVAL FAILS, LET'S PRINT THE ERROR...
print $@;
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You've declared a new lexical variable $newrate that only exists for the duration of the eval block (and hides the previously declared one in the enclosing scope).

You also always print $@. Rather than using eval, probably what you really wanted was:

my $newrate = 0;
if( $second_currency == 0 ) {
  warn "Second currency was zero\n";
} else {
  $newrate = $first_currency /  $second_currency;
}
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