Thanks, that kickstarted my brain.

I'm too old-school to do it your way but I did it this way in the end:

sub get_filehandle {
    my $self = shift;
    $self->{file} = shift;
    open( my $fh, '<', $self->{file} )
      or die "can't open $self->{file}";
    return $fh;
}

sub get_lines {
    my $self = shift;
    $self->{file} = shift;
    ### get the filehandle if we don't already have one
    unless ( $self->{file_handle} ) {
        $self->{file_handle} = $self->get_filehandle( $self->{file} );
    }
    if ( my $line = readline( $self->{file_handle} ) ) {
        return $line;
    } else {
        return;
    }
}
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