If you can provide the package name as a parameter then you can:

use strict;
use warnings;

package Foo;

sub Bar {
}

package main;

sub test {
    my ($package, $function_ref) = @_;

    no strict;
    for my $key (keys %{"${package}::"}) {
        my $ref = ${"${package}::"}{$key};
        if (defined $ref && \&$ref eq $function_ref) {
            print "${package}::$key\n";
        }
    }
}

test ('Foo', \&Foo::Bar);
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Prints:

Foo::Bar
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