It seems that the constructor does not accept values ​​in the scientific format, e.g. "1e20". To get around this limitation, I wrote a function that converts floats represented by a scientific format string to floats represented by a string as a decimal fractional number. You can use the output of this function as the input to the constructor.

<?php
use BCMath\Number;

$num = -123e-40;   // can be a float or string variable, because...
$strNum = sci2dec( $num );  // ...there is an internal float to string conversion
$L = new Number( $strNum );
var_dump( $L );

function sci2dec( string $s ): string
{
    if( !preg_match( '/^([+-]?)(\d+(?:\.\d+)?)[eE]([+-]?\d+)$/', $s, $m ) )
        return $s; // it's not sci format, return unchanged
    [ $sign, $mantissa, $exp ] = [ $m[1], $m[2], (int)$m[3] ];
    [ $int, $frac ] = str_contains( $mantissa, '.' ) ? explode( '.', $mantissa, 2 ) : [ $mantissa, '' ];
    $digits = $int . $frac;
    $exp -= strLen( $frac );
    if( $exp >= 0 )
    {
        $digits .= str_repeat('0', $exp);
        $dec = lTrim($digits, '0');
        return $sign . ( $dec === '' ? '0' : $dec );
    }
    $pos = strLen( $digits ) + $exp;
    if( $pos > 0 ) // kropka w środku
        return $sign . subStr( $digits, 0, $pos ) . '.' . subStr( $digits, $pos );
    return $sign . '0.' . str_repeat( '0', -$pos ) . $digits;
}
?>

Output is:

object(BcMath\Number)#2 (2) {
  ["value"]=>
  string(43) "-0.0000000000000000000000000000000000000123"
  ["scale"]=>
  int(40)
}