# 1210614-2.pl
package MyClass {
    use Moose;
    has FileName => (is => 'rw', isa => 'Str', trigger => \&SetFileName);
    sub SetFileName {
        my $self = shift;
        my $arg = shift;
        warn "I am here with $arg";
    }
};

package main {
    my $obj = MyClass->new;
    $obj->SetFileName('/foo/bar');
};