OK. Then how about this? 
package Levels::Level_IV;

use strict;
use warnings;
use Math::RungeKutta;
...

sub level_iv {
    my ( $args_ref, $R, $factor, $level_iii_ref ) = @_;

    ...

    for my $i ( $t..$t_final ) {

       ...

       ( $t, $dt, @y_med ) = rk4_auto( \@y_med, \&_dydt, $t, $dt, $eps
+ );
    }

    ...

    sub _equation_air {
        my ( $args_ref, $level_iii_ref, $y ) = @_;

#I tried these options
#
#    my $_equation_air = sub {
#        my $y = shift;
#    local *_equation_air = sub {
#        my $y = shift;

        #for easy calculations make copies of the parameters (not alia
+ses)
        my arguments for formula from $args_ref and $level_iii_ref

        #differential equation for mass
        my $dydt = some formula;

        return $dydt;
    };
    
    ...

    #differential equation for fugacity at start and end times
    sub _dydt {
        my ( $t, @y ) = @_;
        my @dydt;
        
        #start differentiation at point y0 (air)
        $dydt[0] = _equation_air( $args_ref, $level_iii_ref, $y[0] );
        
        #tried these
        #$dydt[0] = _equation_air($y[0]);   #local
        #$dydt[0] = $_equation_air->($y[0]);#lexical
        
        ...

        return @dydt;
    }
    
    ...
    
    return some hash;
}

[download]
Failures are described in another post earlier if I try lexical or local.

In reply to Re^7: Closures and undefined subs by Anonymous Monk
in thread Closures and undefined subs by Anonymous Monk

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