Perhaps not as fast as printf, but pretty fast, and not using anything special:

sub round {

    my ($nr,$decimals) = @_;
    return (-1)*(int(abs($nr)*(10**$decimals) +.5 ) / (10**$decimals))
+ if $nr<0;
    return int( $nr*(10**$decimals) +.5 ) / (10**$decimals);

}
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strange that no-one has posted this, seems like the most straightforward idea to me...