Perl> 
sub mod{
    my( $a, $n ) = @_; 
    return $a - $n * int( $a / $n ); 
};;

Perl> print mod( 4.5, 1.25 );;    ## prints 0.75
Perl> print mod( 4.5, -1.25 );;   ## prints 0.75
Perl> print mod( -4.5, -1.25 );;  ## prints -0.75
Perl> print mod( -4.5, 1.25 );;   ## prints -0.75