Found a way to round it safely:

my $frac = shift;
if ($frac =~ /\..{5,}000.$/) {
    $frac =~ s/.$//;
}
elsif ($frac =~ /(\..{5,}999)(.)$/) {
    $frac = $frac + ( (10-$2) . "E-" . length($1));
}
print "rounded to $frac";
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note: $frac can be an integer; $frac must be positive