This other answer, by wrvhage, has the great property of working correctly for values like 3.005 rounded to two places (where sprintf is not). However, it needs a couple of tweaks to work for negative numbers.

sub stround
{
    my( $n, $places ) = @_;
    my $sign = ($n < 0) ? '-' : '';
    my $abs = abs $n;
    $sign . substr( $abs + ( '0.' . '0' x $places . '5' ), 0, $places 
++ length(int($abs)) + 1 );
}
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